Andrew Sullivan has this great link to a study of a year's worth of sexual hook-ups at "Thomas Jefferson" High — my own HS!
This "Jefferson High" was almost all white, in a mid-sized city an hour from the nearest big metro area. A few more than half the kids had had sex — 573.
This study has implications for how to stop sexually transmitted disease. In adult populations, there are "hubs," or key individuals who spread disease. At Jefferson, there weren't — although more than half of the kids who had had sex were part of a huge chain of potential infection:
The most striking feature of the network was a single component that connected 52 percent (288) of the romantically involved students at Jefferson. This means student A had relations with student B, who had relations with student C and so on, connecting all 288 of these students.