Yeah that's because highschool is the first years of adulthood, not the last years of childhood.

16 year old adult women want to fuck because they're biologically ready to become married mothers. The way to keep them from premarital sex is the same as ever: have them marry very shortly after their sex instinct appears, like, within a few years.

It's literally unfair to expect women to wait til their 20s to have sex, they've never done it in all of history and they're not about to start. Cultures that delay motherhood into the late 20s will surely shrink, be conquered, and vanish. They can't meet population replacement rates.

For a woman, getting married at 20 should be seen as "dangerously late". Risking spinsterhood. That's how you keep them from having sex before marriage. Early marriage.