I have experience of giving PSHE talks about sex and relationships to Y11 at Independent schools. I have learnt a lot from them! Seriously, it is incredible the reasons they give for having sex - exercise and money amongst the answers. Love is usually very low on the list.
Contraception is very well understood by them but being careful when drunk and not thinking about the consequences is still a problem.
Teenage pregnancy is a huge problem but, interestingly, amongst independent school girls, it is not the problem that it is in the state sector and so we also need to look at social and other reasons.
Of course the STI time bomb is not prevented by the contraceptives that girls can be responsible for themselves, but that's another topic.