“First base, second base, third base, home run,” Al Vernacchio ticked off the classic baseball terms for sex acts. His goal was to prompt the students in Sexuality and Society — an elective for seniors at the private Friends’ Central School on Philadelphia’s affluent Main Line — to examine the assumptions buried in the venerable metaphor. “Give me some more,” urged the fast-talking 47-year-old, who teaches 9th- and 12th-grade English as well as human sexuality. Arrayed before Vernacchio was a circle of small desks occupied by 22 teenagers, six male and the rest female — a blur of sweatshirts and Ugg boots and form-fitting leggings.

“Grand slam,” called out a boy (who’d later tell me with disarming matter-of-factness that “the one thing Mr. V. talked about that made me feel really good was that penis size doesn’t matter”).

“Now, ‘grand slam’ has a bunch of different meanings,” replied Vernacchio, who has a master’s degree in human sexuality. “Some people say it’s an orgy, some people say grand slam is a one-night stand. Other stuff?”

“Grass,” a girl, a cheerleader, offered.

“If there’s grass on the field, play ball, right, right,” Vernacchio agreed, “which is interesting in this rather hair-phobic society where a lot of people are shaving their pubic hair — ”

“You know there’s grass, and then it got mowed, a landing strip,” one boy deadpanned, instigating a round of laughter. While these kids will sit poker-faced as Vernacchio expounds on quite graphic matters, class discussions are a spirited call and response, punctuated with guffaws, jokey patter and whispered asides, which Vernacchio tolerates, to a point.

Vernacchio explained that sex as baseball implies that it’s a game; that one party is the aggressor (almost always the boy), while the other is defending herself; that there is a strict order of play, and you can’t stop until you finish. “If you’re playing baseball,” he elaborated, “you can’t just say, ‘I’m really happy at second base.’ ”

A boy who was the leader of the Young Conservatives Club asked, “But what if it’s just more pleasure getting to home base?” Although this student is a fan of Vernacchio’s, he likes to challenge him about his tendency to empathize with the female perspective.

“Well, we’ve talked about how a huge percentage of women aren’t orgasming through vaginal intercourse,” Vernacchio responded, “so if that’s what you call a home run, there’s a lot of women saying” — his voice dropped to a dull monotone — ‘O.K., but this is not doing it for me.’ ”

In its breadth, depth and frank embrace of sexuality as, what Vernacchio calls, a “force for good” — even for teenagers — this sex-ed class may well be the only one of its kind in the United States. “There is abstinence-only sex education, and there’s abstinence-based sex ed,” said Leslie Kantor, vice president of education for Planned Parenthood Federation of America. “There’s almost nothing else left in public schools.”

Across the country, the approach ranges from abstinence until marriage is the only acceptable choice, contraceptives don’t work and premarital sex is physically and emotionally harmful, to abstinence is usually best, but if you must have sex, here are some ways to protect yourself from pregnancy and disease. The latter has been called “disaster prevention” education by sex educators who wish they could teach more; a dramatic example of the former comes in a video called “No Second Chances,” which has been used in abstinence-only courses. In it, a student asks a school nurse, “What if I want to have sex before I get married?” To which the nurse replies, “Well, I guess you’ll just have to be prepared to die.”

In settings outside schools, the constraints typically aren’t as tight. Bill Taverner, director of the Center for Family Life Education for Planned Parenthood of Greater Northern New Jersey, said that his 11 educators are usually given the most freedom with so-called high-risk youth, those in juvenile detention, or who live in poor neighborhoods with high teen-pregnancy rates. “I wish I could say it was for positive reasons,” he said, “but it’s almost as if society has just kind of thrown up their hands and said, ‘Well, these kids are going to have sex anyway, so you might as well not hide anything from them.’ ”

Sex education in America was invented by Progressive Era reformers like Sears, Roebuck’s president, Julius Rosenwald, and Charles Eliot, the president of Harvard University. Eliot, according to Kristin Luker, author of the book “When Sex Goes to School,” concluded that sex education was so important that he turned down Woodrow Wilson’s offer of the ambassadorship to Britain to join the first national group devoted to promoting the subject. Eliot was one of the so-called social hygienists who thought that teaching people about the “proper uses of sexuality” would help stamp out venereal disease and the sexual double-standard that kept women from achieving full equality. Proper sex meant sex between husband and wife (prostitution was then seen as regrettable but necessary because of men and their “needs”), so educators preached about both the rewards of carnal contact within marriage and the hazards outside of it.

It wasn’t until the 1960s and 1970s that the pill, feminism and generational rebellion smashed the cultural consensus that sex should be confined to marriage. And for a “brief, fragile period” in the 1970s and early 1980s, writes Luker, a professor of sociology and of law at U.C. Berkeley, “opinion leaders of almost every stripe believed sex education was the best response to the twin problems of teenage pregnancy and H.I.V. AIDS.” It was around this time that the Unitarian Universalist Association started its famously sex-positive curriculum, About Your Sexuality, with details about masturbation and orgasms and slide shows of couples touching one another’s genitals. (The classes are still going strong, though in the late 1990s, the program was replaced with another one without explicit images called Our Whole Lives, a joint project of the U.U.A. and the United Church of Christ.)

Back then, even public schools taught what came to be called “comprehensive sex education,” nonjudgmental instruction on bodies, birth control, disease prevention and “healthy relationships” — all geared to helping teenagers make responsible choices, one of which might be choosing to become sexually intimate with someone. But by the end of the 1980s, sex ed had taken its place in the basket of wedge issues dividing the right and left. This created the opening for abstinence instruction (the word “abstinence” wasn’t part of the sex-ed vernacular until the 1980s) to bulldoze any curriculum that didn’t treat sex as forbidden for teenagers. But Kantor and many others in the field remain “comprehensive sex ed” believers. To them, the license Vernacchio has to roam the sexual landscape is almost unimaginable.

Sitting in the conference room at Friends’, a tall, striking girl told me after class one day last winter that she was on the verge of getting involved with someone she really liked but was hesitating because she knew he had a reputation for juggling multiple girlfriends. The girl, who’d had sex twice in 11th grade with a boy she later discovered was sleeping around, wanted to be monogamous with the new guy but didn’t know how to broach it with him. (She was one of 17 students in Sexuality and Society who spoke to me privately; while Vernacchio is happy to discuss any personal information the kids bring up, he doesn’t seek it.)

Another young woman, who tended to treat her tiny desk in Vernacchio’s class as a lounger, flinging her legs out toward the center of the room, told me that she enjoyed sex for its own sake — the way guys do, as she put it. While she could express this with some bravado now, she came into Sexuality and Society in the beginning of the year uneasy about this aspect of herself, she said. A third girl, who called herself a “really anxious person,” still got choked up discussing a false rumor someone wrote on Facebook last fall: that she’d drunkenly offered oral sex to a boy at a party, who, as it happened, also was enrolled in Sexuality and Society. That young man, who didn’t post the lie and was predictably unfazed by it, had fallen for another classmate. That girl was equally besotted, but because they were in “sex class,” the couple always positioned themselves across the room from each other, never side by side; otherwise, they told me, they’d feel like animals in a zoo. Not that the pair weren’t still on display. With the exception of Vernacchio, everyone knew their status and found it impossible not to notice them locking eyes periodically, smiling briefly, before she’d duck her head, push her long, shiny hair behind her ears and turn her gaze back to her teacher.

“Mr. V. takes every question seriously,” another girl, the student-council vice president, told me. “You never feel like it’s the wise sexuality master preaching to the young.” Yet Vernacchio also doesn’t give off the vibe that he wants to be young, or imagines that he still is. His attire every day for the two weeks I attended the class in February was a sweater vest over a button-down shirt and tie, except for Valentine’s Day, when he shed the vest for a ruby red shirt and a tie decorated with hearts. That day Vernacchio gave all of his students brightly colored origami hearts he made himself; the members of Sexuality and Society reciprocated by sending him a singing Valentine (a “Glee”-worthy rendition of “Everytime We Touch,” by the boys’ barbershop choir).

Vernacchio is nothing so much as a mensch. Gay, with a partner of 17 years, he has ruddy cheeks, a quick smile and a plane of brown hair overhanging his brow, from which he must regularly wipe away sweat during intense discussions. He lectures with plainspoken authority while also conveying a deep curiosity about his subject — the consummate sex scholar.

During a lesson about recognizing your “crumble lines” — comments that play to your vulnerability and may make you “act against your values” — Vernacchio, a self-described “short, round, hairy guy” who struggles with body-image issues, revealed his own tendency to fall for anybody who compliments his appearance: “You say you think I’m pretty. I’ll do anything for you.” He was exaggerating a bit for effect, but the poignancy of the self-disclosure wasn’t lost on the class.

Friends’ Central, a Quaker prep school that prides itself on both its academic rigor and its ethic of social responsibility, is tucked away in the bucolic hills of suburban Philadelphia. Vernacchio joined the school’s English department in 1998, and when, three years later, he asked to start Sexuality and Society, administrators were delighted. “He teaches at the very highest level,” said David Felsen, who in June retired as headmaster of the school after 23 years. Because Vernacchio was such a gifted instructor, Felsen said, he didn’t worry about parents’ reactions. And in fact, Vernacchio says that no one has ever complained or even voiced reservations about something he discussed in class.

The parents I spoke to — ranging from a father who said he loves his son “to pieces” but wishes he knew him better to a mom who gets frequent updates from her daughter now in college — seemed grateful for the class. “My daughter is sometimes private,” another mother said, “and I appreciate that there was another place she could go to get good, healthy information.” Early in the year, Vernacchio gives an assignment asking students to interview a parent about how he or she learned about sex, and the father said his son handled it with aplomb: “He was very natural, and I’m the one thinking, This is embarrassing. He was a lot more mature about the conversation than I was.”

Sexuality and Society begins in the fall with a discussion of how to recognize and form your own values, then moves through topics like sexual orientation (occasionally students identify as gay or transgender, Vernacchio said, but in this particular class none did); safer sex; relationships; sexual health; and the emotional and physical terrain of sexual activity. (The standard public-school curriculum sticks to S.T.I.’s and contraceptive methods, and it can go by in a blink; in a Kaiser Family Foundation survey, two-thirds of principals said that the subject was covered in just several class periods.) Vernacchio also teaches a mandatory six-session sexuality course for ninth graders that covers some of the same material presented to the older kids, though less fully.

The lessons that tend to raise eyebrows outside the school, according to Vernacchio, are a medical research video he shows of a woman ejaculating — students are allowed to excuse themselves if they prefer not to watch — and a couple of dozen up-close photographs of vulvas and penises. The photos, Vernacchio said, are intended to show his charges the broad range of what’s out there. “It’s really a process of desensitizing them to what real genitals look like so they’ll be less freaked out by their own and, one day, their partner’s,” he said. What’s interesting, he added, is that both the boys and girls receive the photographs of the penises rather placidly but often insist that the vulvas don’t look “normal.” “They have no point of reference for what a normal, healthy vulva looks like, even their own,” Vernacchio said. The female student-council vice president agreed: “When we did the biology unit, I probably would’ve been able to label just as many of the boys’ body parts as the girls’, which is sad. I mean, you should know about the names of your own body.”

Vernacchio is aware that his utter lack of self-consciousness in conversing about sexual matters is unusual. “When God was passing out talents,” he likes to say, “I got ease in talking about sex.” But any plan of God’s, whom Vernacchio, a practicing Catholic, often references, was nudged along by two earthly happenings. “As a little kid,” Vernacchio said, “I got pegged as a good public speaker, so I started narrating all the school plays and reading at church; I got over the fear of speaking really early.” Then, around age 12, he started to research sex, having known from kindergarten that he was different in a “way that had to do with boys and girls.” He looked up homosexuality in the family dictionary, then took to going to libraries and planting himself in the sexuality section of the stacks. “I used to have the Dewey-decimal number for homosexuality memorized.” He was entirely on his own. There was no discussion of being gay at Vernacchio’s all-boys school; none from his parish priest, who at the end of sermons offered a prayer for “veterans of foreign wars, people who live near nuclear power plants and homosexuals”; and not from his parents, either, even after he came out to them at 19. Indeed, one night several years later, his mom was doing dinner dishes at the sink and his dad was plopped on the couch a few feet away in their tiny South Philly house, and Vernacchio mustered the courage to tell them that he was happily dating someone. “My mom never turned around, never reacted in any way, and my dad turned to me, didn’t miss a beat and said, ‘Whatever happened to the metric system?’ ”

It was drummed into him as a human-sexuality master’s student, Vernacchio said, to never be explicit merely for the sake of being explicit: have a rationale for every last thing you say. Which occurred to me one day listening to him answer an anonymous question — there’s a box on the bookshelf where students can drop them — about whether a girl’s urge to urinate during intercourse might be a precursor to female ejaculation. He laid out a plethora of explanations for the feeling, everything from anxiety about having sex to a bladder infection to the possibility that the young woman was getting “some really good G-spot stimulation” and in fact verging on ejaculation.

“If kids are starting to use their bodies sexually, they should know about their potentialities,” Vernacchio told me later. “It’s O.K. that boys ejaculate, that’s totally normalized” — wet dreams have been standard fare for middle-school health class for decades — “but girls, gross! Girls will think they’re peeing themselves, and it’s really shameful.”

“I just love this class — you can ask anything,” a member of the girls’ basketball team told me one day in February. She wears her long blond hair in two braids and shyly divulged that she was in love with her boyfriend of eight months. “You may not be able to get the best information on the Internet, but you can ask Mr. V., and he’ll either know it or ask his sex-ed friends,” she said, referring to a sex-educators’ e-mail list that Vernacchio consults.

Two boys who told me they’d been masturbating to Internet porn since middle school said they found themselves disoriented at the real-life encounters they had with girls, but Vernacchio helped them grasp the disjuncture. Pornography “gives boys the impression that the girl is there to do any position you want, or to please you, or to, you know, role-play to your liking,” one of them said. “But yesterday, when Mr. V. said there is no romanticism or intimacy in porn, porn is strictly sexual — I’d never thought about that.”

One young man in the class told me he had intercourse with 10 girls, but he was a relative outlier. While most of the students had had intercourse — 70 percent of teenagers do so by their 19th birthday, according to the Gutt­macher Institute — only 4 of the 17 I spoke with reported having three or more partners; 10 had had one or two; the other three were virgins.

But the numbers fail to capture the variation within the sexual histories. Of the two girls with more than two partners, one was the girl who appreciated purely sexual encounters. The other told me that during the summer before ninth grade, she was raped one night on a beach by a stranger. She told no one, she said, and she subsequently got together with a number of boys in what she now saw as a misguided effort to “take control” of her sexuality.

As to whether his class encourages teenagers to have sex — a protest perennially lodged against even basic sex ed (though pretty firmly disproved by research) — Vernacchio said that he portrays sex in all its glory and complications. “As much as I say, ‘This is how orgasms work, and they’re really cool,’ I say there’s a lot of work to being in a relationship and having sex. I don’t think I have the power to make sex sound so enticing that kids are going to break through their self-esteem issues or body stuff or parental pressures or whatever to just go do it.” And anyway, Vernacchio went on, “I don’t necessarily see the decision to become sexually active when you’re 17 as an unhealthy one.” His goal is for young people to know their own minds, be clear about what they do and don’t want and use their self-knowledge to make choices.

To that end, he spends one class leading the students through a kind of cost-benefit analysis of various types of relationships, from friendship to old-school dating to hookups. When he asked his students about the benefits of hookups, the kids volunteered: “No expected commitment,” “Sexual pleasure” and “Guarding emotions,” meaning you can enjoy yourself without the messiness of attachment.

“Yep,” Vernacchio said, “sometimes a hookup is all you want.” Then he pressed them for drawbacks.

“You may not be able to control your emotions,” someone called out.

“O.K.,” Vernacchio said approvingly. “What else?”

“It’s confusing,” said the student-council vice president.

“Yeah,” Vernacchio said, explaining that two people may have different ideas about what it means to hook up, which is why communication is so important. (“If you can’t talk about it, you probably shouldn’t be doing it,” he says.)

“People saying, ‘Oh, she’s a slut,’ ‘Oh, he’s a man-whore,’ ” floated a boy who described himself to me as a “lonesome outcast” until 11th grade, when he finally started to make friends. “I guess for women it’s usually seen as more of a bad thing.”

“Right,” Vernacchio agreed, “but there’s pressure on guys too. Guys get the, ‘Oh, yeah, he’s a player,’ but what if you’re really not? And then you feel pressure to maintain that.”

Vernacchio rarely misses a chance to ask his students to examine gender bias in their sexual attitudes or behavior. The girl who “admitted” to liking sex as much as boys did said that Vernacchio’s consistent affirmation of the variety of sexual preferences (“Guys aren’t necessarily naturally hornier than girls — there’s a huge social piece of this,” he told the class) helped her shake her sense of deviance and shame. In fact, she felt confident enough to debate her point of view in class with the girl who was nervous about embarking on a relationship with the guy known to be promiscuous. That young woman told me she’d been moved by the exchange: “I’m like, ‘That is nasty to hook up with someone for one night.’ She was like, ‘Well, I don’t care, sometimes I don’t want a relationship.’ We were going back and forth, but then I had to respect her. Before I took this class, I probably would’ve thought she was a whore, but she knows what she wants. That’s not something I want, but it doesn’t make her wrong, it doesn’t make me wrong.”

Above all else, what Vernacchio can do that his colleagues envy is to simply assume the pleasure of sex and directly address it with unharried ease. During one class, he handed out a worksheet with the five senses printed along the top and asked the students to try and list sexual activities that optimized each. (There were examples to prod their thinking: under hearing, for instance, was “listening to your partner read an erotic story.”) While Vernacchio knew the exercise would be a challenge for the kids — and he didn’t expect them to share their answers — its purpose was to open their minds to a broader sexuality.

Regarding the statistic that Vernacchio alluded to earlier — that 70 percent of women do not orgasm through vaginal penetration alone — one boy exclaimed when we talked, “That shocked me, a lot.” The other boys also told me they’d been in the dark about the mysteries of female sexual satisfaction. “I think I sort of knew where the clitoris was, but I didn’t know it was, like, under something,” one said. Another declared, “It’s almost like a wake-up call.” He paused. “To not just please yourself.”

The female students were nearly equally surprised. “I always thought, Is it weird that I don’t get an orgasm from, you know, just like vaginal penetration?” said a girl who’d had intercourse with one boy, though she’d had orgasms before that from being touched genitally. “It was comforting to hear that for most people it doesn’t happen. I mean, I’d heard it, but it was nice hearing it from Mr. V., who knows so much about it, and other people saying, ‘Yeah, yeah, that’s right.’ ”

Not that information was always power for these young women. One girl said that while she could advise her boyfriend on how to increase her pleasure, she wouldn’t, because he’s “very insecure” about his lack of experience. Another estimated she’d had only two orgasms with her boyfriend of longstanding, each during intercourse, though she climaxes on her own through masturbation. Somehow, when she and her boyfriend “do anything, we just end up having sex,” she said, seeming both a little perplexed by the situation, and a little afraid to make waves.

Who gives oral sex to whom is common fodder for Vernacchio’s gender-parity conversations. All but one of the students told me they’d had it, but sometimes only once or twice, and the vast majority within monogamous relationships.

Although Vernacchio encourages students to think about fairness, he certainly doesn’t encourage a direct quid pro quo for oral sex — and the girls, the main givers, were not terribly enthused about being the recipients. “[My boyfriend] completely offered, and I did not want that,” one said. Another agreed: “It just creeps me out.” None were thrilled about performing it, either, and they seemed to be wrestling — in thought and deed — with why they continued to do so. “I do think girls like to take care of people,” the student-council V.P. mused, “and I know that just sounds horrible, like you should send me right back to the ’50s, but my mom is like the most liberal woman I know and still is so happy to make food for people. To some extent, women are just more people-pleasers than men.” One girl said she’d come up with “tricks” to make giving oral sex more enjoyable for her, and that she’d set “strict rules” for herself: “I only do it if they do something on me first, and it has to be below the belt.” And another said she doesn’t enjoy cunnilingus, but taking the personal is political to heart, she asked her boyfriend to do it anyway: if she was expected to service him orally, he should have to return the favor.

All the boys said that Vernacchio had increased their sensitivity to the girls. One recounted how in an effort to consider his girlfriend’s feelings he’d asked her if she was willing to give him oral sex — none of that pushing her head down in the heat of the moment — and she’d considered it for an excruciating hour. Or maybe it just felt like that. “Do you have to think about it this long?” he finally pleaded. Eventually, she agreed.

Pleasure in sex ed was a major topic last November at one of the largest sex-education conferences in the country, sponsored by the education arm of Planned Parenthood of Greater Northern New Jersey. “Porn is the model for today’s middle-school and high-school students,” Paul Joannides said in the keynote speech. “And none of us is offering an alternative that’s even remotely appealing.”

Joannides, who is 58, made sex education his life’s work following the success of his sex manual for older teenagers and adults called, “The Guide to Getting It On.” Lauded for its voluminous accuracy and wit, the 900-plus-page paperback took him 15 years to research and write. Joannides argues that pornography can be used as a teaching tool, not a bogeyman, as is apparent in a short Web video he made called “5 Things to Learn About Lovemaking From Porn.” “In porn,” he affably lectures, “sex happens instantly: camera, action, crotch. . . . In real life, the willingness to ask and learn from your partner is often what separates the good lovers from those who are totally forgettable.” (Another of Joannides’s assertions is that the best way to reach heterosexual boys — who he believes are the most neglected in the current environment — is to play to their desire for “mastery,” because by middle school, they’ve thoroughly absorbed that to be a man is to be a stud.)

One of sex educators’ big problems, Joannides told the New Jersey audience, is that they define their role as the “messengers of all the things that can go wrong with sex.” The attention paid to S.T.I.’s, pregnancy, rape and discrimination based on sexual orientation, while understandable, comes at a cost, he says. “We’re worrying about which bathrooms transgender students should use while teens are worrying whether they should shave all the way or leave a landing strip,” he said. “They’re worrying if someone special will find them sexually attractive, whether they will be able to do it as well as porn, whether others have the same kind of sexual feelings they do.”

In other words, as much as Joannides criticizes his opponents on the right, he also tweaks the orthodoxies of his friends on the left, hoping to spur them to contemplate how they themselves dismiss pleasure. His main premise is that young people will tune out educators if their real concerns are left in the shadows. And practically speaking, pleasure is so braided through sex that if you can’t mention it, you miss chances to teach about safe sex in a way that young people can really use.

For instance, in addition to pulling condoms over bananas — which has become a de rigueur contraception lesson among “liberal” educators — young people need to hear specifics about making the method work for them. “We don’t tell them: ‘Look, there are different shapes of condoms. Get sampler packs, experiment.’ That would be entering pleasure into the conversation, and we don’t want that.”

While the conference attendees couldn’t have agreed more with Joannides about what should be taught in schools, much of the crowd thought he was deluded to imagine they could ever get away with it. Back in 1988, Michelle Fine, a professor of social psychology at the City University of New York, wrote an article in The Harvard Educational Review called “Sexuality, Schooling and Adolescent Females: The Missing Discourse of Desire.” In it, she included the comments of a teacher who discouraged community advocates from lobbying for change in the formal curriculum. If outsiders actually discovered the liberties some teachers take, Fine was told, they’d be shut down.

More than two decades later, at the conference, an educator from Pennsylvania told me that one school asked her to teach a sex-ed class but forbade her to use the words “sex, ” “sexy” or “tampon.” (She declined.) A chipper young Unitarian sex educator from Brooklyn, Kirsten deFur, who led a workshop titled “Don’t Forget the Good Stuff,” gave tips on how her colleagues could avoid uttering the words “pleasure” and “orgasm.” “Ask open-ended questions about what feels good,” deFur recommended. And, she added, the P-word might even be acceptable in the proper context: “If you have healthy sex, it’ll be more pleasurable,” an instructor might dare to say.

That more expansive sex education has to be done in code was something I came across repeatedly. A veteran advocate in the field gave me a short list of teachers to contact who might be willing to talk to me but then warned, “I don’t know if any of them are going to want to have what they’re doing out there.”

“What if our kids really believed we wanted them to have great sex?” Vernacchio asked near the end of an evening talk he gave in January primarily for parents of ninth graders who would attend his sex-ed minicourse. “What if they really believed that we want them to be so passionately in love with someone that they can’t keep their hands off them? What if they really believed we want them to know their own bodies?”

Vernacchio didn’t imagine that his audience, who gave him an enthusiastic ovation when his presentation ended, wanted their 14- and 15-year-olds to go out tomorrow and jump into bed or the backseat. Sex education, he and others point out, is one of the few classes where it’s not understood that young people are being prepared for the future.

Sex, of course, can come with emotional confusion and pain, and be enmeshed with violence, which Michelle Fine knows well. She said that what all adolescents crave is a “safe space” to pull apart and ponder the stew of relationships and sexual activity — including intimacy and desire and betrayal and coercion.

Vernacchio’s classroom is such a setting. Owing partly to his devotion to his job, partly to the individual relationships he starts developing with students in ninth grade as their English or sex-ed instructor or adviser, he looks out at a roomful of people whom he really knows, and who depend on him for discerning and generous counsel. This was especially true for the young woman who was raped — she told Vernacchio about the assault before anyone else at Friends’ — as well as the girl who was undone by her scorching on Facebook. She relied on Vernacchio all year for support, she said.

For every single question that Vernacchio pulls out of his anonymous question box about female ejaculation, there are 10 like these: How do you handle your insecurities in a relationship? How do you stop worrying about being cheated on? How do you know when it’s time to break up? How do I talk to my partner about wanting to spend more time together without being annoying? Watching how closely the students attended to Vernacchio’s often lengthy answers was a moving reminder of how young 17- and 18-year-olds are.

“As a society, we always tell kids, ‘Work hard, just focus on school, don’t think about girls or guys — you can worry about that stuff later, that stuff will work itself out,’ but the thing is, it doesn’t,” said a boy who had told me he had a disconcerting one-nighter with a girl he’d talked to only electronically. The class taught him to be more cautious about choosing the right time with the right person, he said, with a forcefulness that didn’t quite cover the hurt in his eyes. “You learn about the psychological after-effects that could happen to you.”

The girl who was contemplating getting serious with a boy, but only if they could be exclusive, told me she finally figured out how to approach the guy after Vernacchio talked in class about the difference between “nagging” and asking for what you want. “I never thought of saying to him, ‘You know, just tell me if you’re having sex with someone else.’ I don’t want to pressure him, but I feel like it would make me comfortable.” This seems like pretty simple stuff, especially for someone who repeatedly called herself “strong,” but somehow it wasn’t until Vernacchio said that it was O.K. to make such forthright requests that she could conceive of it.

“The campaign for abstinence in the schools and communities may seem trivial, an ideological nuisance,” Michelle Fine and Sara McClelland wrote in a 2006 study in The Harvard Educational Review, “but at its core it is . . . a betrayal of our next generation, which is desperately in need of knowledge, conversation and resources to negotiate the delicious and treacherous terrain of sexuality in the 21st century.”

It’s axiomatic, however, that parents who support richer sex education don’t make the same ruckus with school officials as those who oppose it. “We need to be there at the school boards and say: ‘Guess where kids are getting their messages about sex from? They’re getting it from porn,’ ” Joannides exhorted. “All we’re talking about is just being able to acknowledge that sex is a good thing in the right circumstances, that it’s a normal thing.”

Of course, sex isn’t all pleasure or all peril, it’s both (and sometimes both at once, though that lesson may have to wait for grad school). Vernacchio has a way of getting at its positive potential without ignoring the fact that, however good sex may feel, it’s sometimes best left off the menu. “So let’s think about pizza,” Vernacchio said to his students after they’d deconstructed baseball. The class for that day was just about over. “Why do you have pizza?”

“You’re hungry,” a cross-country runner said.

“Because you want to,” Vernacchio affirmed. “It starts with desire, an internal sense — not an external ‘I got a game today, I have to do it.’ And wouldn’t it be great if our sexual activity started with a real sense of wanting, whether your desire is for intimacy, pleasure or orgasms. . . . And you can be hungry for pizza and still decide, No thanks, I’m dieting. It’s not the healthiest thing for me now.

“If you’re gonna have pizza with someone else, what do you have to do?” he continued. “You gotta talk about what you want. Even if you’re going to have the same pizza you always have, you say, ‘We getting the usual?’ Just a check in. And square, round, thick, thin, stuffed crust, pepperoni, stromboli, pineapple — none of those are wrong; variety in the pizza model doesn’t come with judgment,” Vernacchio hurried on. “So ideally when the pizza arrives, it smells good, looks good, it’s mouthwatering. Wouldn’t it be great if we had that kind of anticipation before sexual activity, if it stimulated all our senses, not just our genitals but this whole-body experience.” By this time, he was really moving fast; he’d had to cram his pizza metaphor into the last five minutes. “And what’s the goal of eating pizza? To be full, to be satisfied. That might be different for different people; it might be different for you on different occasions. Nobody’s like ‘You failed, you didn’t eat the whole pizza.’

“So again, what if our goal, quote, unquote, wasn’t necessarily to finish the bases?” The students were gathering their papers, preparing to go. “What if it just was, ‘Wow, I feel like I had enough. That was really good.’ ”

Mark Webber won the Brazilian Grand Prix for the second time today, the seventh victory of his F1 career, but crucially it was his first win of the season and it came about because of a rare technical problem for his team mate Sebastian Vettel. The World champion rolled in second having survived most of the race with a gearbox problem, which forced him to short shift in second and third gears and eventually to use only the higher gears. Jenson Button came through after an intense race long battle with Fernando Alonso to claim his 12th podium finish of More…