'I think the posters who say they know their children or their children's friends would never do something like the OPs daughter are being VERY naive.' Not naive, but very aware and watchful. We are abroad, his friends all go to the same international school, and my ds doesn't get out much, he goes to school on the bus and comes home on the bus and is not allowed off campus at lunchtimes.
He does 2 or 3 afterschool activities which are all supervised, homework and at weekends sleeps a lot; he may go over to a friends house to sleep over, but the Mums are much the same as me and well up to speed.
As for teaching them about contraception - he's been told he has to wear at least 40 condoms at a time by which time the urge will have passed! He knows damn well he has to wear a condom, but equally having been pegged as one of the geeky crowd at school, peer pressure won't at the moment come into this. I also know he doesn't have girlfriend yet because he still hates having a shower and cleaning his teeth, and he'd be doing both voluntarily if he had rather than me threatening him with a hose in the garden.