Junior year at my school in Michigan. I can tell you around here everyone talks bout sex and how much they have it; these people are lying. The people that do have sex don't talk about it 90% of the time.
Now, drugs and alcohol on the other hand... 1/3 of the school's population is on drugs (ranging from marijuana to crack-cocaine) or getting drunk every weekend.
There are a few things I can tell you about high school.
1. Peer pressure is the number one thing you'll run into. "Dude, just do it what are you a pussy?" Often leads to a bad decision that could get you in trouble. The best thing to do is ignore it. Think ahead and ask yourself how this will affect your future.
2. Just like everyone changes when they get into high school, everyone changes when they get out, too. My teacher's high school friend slapped a girl in the face with his penis while smoking a joint in the middle of gym class, and had done stuff like that regularly throughout high school. He got his shit together after graduations and became one of the top US military snipers in that time period.
3. Fights happen pretty regularly between people that (a) want people to think they're tough because they're insecure about themselves, or (b) don't think about the consequences that follow. Word of advice - if you have a problem with somebody, either take care of it responsibly with a mediator (or, in my school, we fill out incident reports that go on record without anybody knowing) or take care of it outside of school.
I know this isn't a thread about what to do in high school, but these are things that happen throughout, and therefore is somewhat sufficient to this thread.