There is a difference between an 18 year old HS student deciding to have sex and a 14 year old HS student deciding to have sex. Only one of them is of the legal age of consent in most western countries. If a child is bound and determined to have sex they will. There is little to be done to prevent it when the child is actively trying to make it happen. However, there are a lot of children (and a 14-16 year old is still a child no matter how much they protest) whom are willing to accept prolonged virginity if the opportunity does not present itself. One of my sisters sought out opportunities to have sex, while my other sister, myself and 2 brothers were willing to wait. Nothing my parents did could prevent her (she ended up nocked up by 19), but their involvement in our lives, and perpetual presence in our home probably kept some of us from having sex earlier than we did.
Furthermore, my wife lost her virginity at 14 and said she felt it was a huge mistake. In fact, most of the women I know who lost their virginity before 18 have told me they wished they hadn't. That's not my perception as a new father, but what women have been telling me for over a decade and I have no reason to believe they lied to me. Many of them did so out of a perception that "everyone is doing it", but felt totally unprepared before, during and after the event. Many of them didn't have sex again for several years after breaking up with their first partner because they didn't feel they could handle it yet (my wife incuded). I realize that the Baby Boomers here in the US believe that they are the first generation to have ever had a good idea, but I though that most /.ers were smarter than that. Cultural norms with regards to age of consent, and age appropriate behavior evolved over time for a reason. They many not be the best, but their is usually a good reason for the norms being what they are/were.
Now, with all of that being said... I don't believe that pushing back the start of classes at the HS by an hour will result in increased teenage sex. The vast majority of students will use the extra hour in the morning to get more sleep. I could see a little more recreational drug use (had a friend who used to get high before school), but it's not like it's going to make kids use drugs that wouldn't have anyway. I've been wanting schools to move start times back for years. I was a morning person in HS, but my older brother was not. I had to get up an extra half an hour early just to get my brother out the door on time. Even as a morning person, I would have appreciated the extra hour of sleep.