The most useful shit I learned about sex, I learned from my mum. Namely, my most absolute, unbreakable, set-in-stone rule when it comes to sex: make sure you can't get your partner pregnant until you know you're DAMN ready to have kids (if you want them at all, which I do not), and ensure that, should you have the misfortune of obtaining a STD, you should take every reasonable measure to ensure that its not spread to your partner.
The shit I got from school was mostly abstinence-only bullshit, aside from my 9th grade health class, which actually gave real, honest stats on the efficacy of various methods of contraception.