I think sex should first be brought up when the child shows interest in knowing where the came from. I first learned about sex when I was six, because I was ready to know. If a child doesn't know anything yet by puberty, then they should be thoughroully informed then; and the kids who already asked should get a more in-depth lesson.
I think at this point, it needs to be stressed that sex is for adults who are in love, and that it is not an activity for children or teenagers. I'd explain why it is dangerous for teens to have sex - without lying. I would also encourage my child to talk to me if they ever had questions about sex, because it's not a taboo subject. I would assure them I wouldn't get angry at them about sex and that I felt it was far more important to talk to me and be safe, than to be scared and unsafe.
Once my child has a serious relationship, I'll help him/her get the proper protection if they seem interested in sex - if they are over the age of 18. If they're underage, I'm going to stress again that sex is not a safe activity for teens, and then get them protction, but I'll be paying a lot more attention to it and to their activities.
Girls aren't allowed to have sleepovers with boys. If my child is gay, then they're only allowed sleepovers with the gender they're not sexually attracted to, lol. On that note, my kid is going to know about homosexuality by puberty. I didn't know about homosexuality until around that time and I am still amazed about that fact to this day. I am totally comfortable with the idea of being gay, lesbian or bi, but I'm not going to push my child to identify their sexuality.
I've completely gotten off tangent, but I think a major point here is that much of sex education responsibility lies with the parents, not the schools.