The twisted, drug-fueled phenomenon that's sweeping Manhattan prep schools

It's 11:30 on a Friday, and a dozen students from Dalton, an elite Manhattan private school, are splayed out on nap rugs, munching graham crackers and sipping boxes of Mott's apple juice. Raffi plays quietly in the background, and a young woman with a maternal voice reads aloud to them from a stack of children's books, starting with The Cat in the Hat.

A few missing details:

It's 11:30 at night; the kids are on the floor of a yellow school bus that's parked not in Manhattan but a desolate lot in Williamsburg; and everyone on the bus, including the storyteller, is in high school. Oh, one more thing: they're all on 5-methoxy-N N-diisopropyltryptamine, better known as the club drug Foxy.

Every two weeks, in some forsaken corner of the city, New York's privileged teenagers go to "Sindergarten," a traveling party for 17-year olds who, for a few carefree hours, want to feel like they're five again. Nursery school-style accessories—snacks, children's music, storybooks, finger-paints—are supplemented with multiple doses of Foxy methoxy, a hallucinogen similar to Ecstasy said to facilitate a childlike sense of wonder with the world.

Since September, Josh estimates, about 300 kids have infantilized themselves at his parties, coming from as far and wide as Riverdale's "Hilltop" academies to the Berkeley Carroll School in Park Slope—and he's struggling to keep up with demandSindergarten parties are the creation of Josh (not his real name), a senior at the Fieldston School in the Bronx suburb of Riverdale. He came up with the idea last summer in the Hamptons, he says, when he and his friends were lamenting the artificiality of their computer-age adolescences.

"We grew up with the Internet, with a thousand channels on TV, with cell phones and Blackberries and iPods and all this technology," he says in a characteristic rapid-fire baritone, taking a pull from his Winston Light outside a sidewalk café on the Lower East Side. "We didn't have real childhoods. This is our way of getting back what we were deprived."

Of course, Josh gets a little more out of Sindergarten than Jungian catharsis: He charges $50 a head for the three-hour event, a fee that students from the day schools across the city happily fork over, often in groups of 10 to 15. After overhead costs (bus rental, gas, snacks, supplies, $100 payment to a scruffy 22-year-old with a commercial driver's license who rents and drives the school bus), he can clear up to $800 for a night's work.

Since September, Josh estimates, about 300 kids have infantilized themselves at his parties, coming from as far and wide as Riverdale's "Hilltop" academies to the Berkeley Carroll School in Park Slope—and he's struggling to keep up with demand. "I service whichever schools show the most interest for that weekend," he explains. Once he's set up an appointment with that school's point man (who must be referred by a fellow sindergartener), Josh discloses the location of the school bus for the night—usually an abandoned lot somewhere in Brooklyn or Queens. He arranges the environmental accoutrements and oversees the activities, but abstains from using Foxy. "I've seen a lot of friends get hospitalized for that shit," he says. When asked if he fears being held accountable for any overdoses on his bus, he becomes shrewdly legalistic. "I don't know of any drug use on the Sindergarten bus—that's idle speculation. What people do before they get on isn't my responsibility. I'm running a fully legal business." A spokesman for the NYPD said the department has never received any complaints about Sindergarten, but added that Foxy arrests and seizures have spiked ten-fold since 2005.

"We all do it," writes an 18-year-old sindergartener, Sarah, in an e-mail interview. "I wouldn't say people are addicted to it, but it'd be hard to let yourself go like that without a little Roxy [a slang term for Foxy] in your system." (Sarah, the daughter of divorced Upper East Side parents who are both active—and highly visible—on the charity circuit, was hesitant to participate in this story, and agreed to do so on the condition that we not use her real name.)

Nevertheless, nothing traumatic has happened so far, and the Sindergarten subculture is thriving. As is wont to happen with teens—even teens pretending to be preteens—a distinct lingo has sprung up: when sindergarteners hook up on the bus (a frequent event, according to Sarah), it's called "playing doctor"; whoever temporarily plays the adult for the "class" is called "teacher"; and if anyone uses a word deemed to be above the 4th-grade level, the offender must go to the back of the bus for a mandatory "ten-minute timeout." Besides "story time," their activities are varied: sing-alongs, show-and-tell, arts and crafts, and whatever games one can play inside a cramped Type A-1 sixteen-seat school bus.