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.