Never trust school lunches, except for the pizza that they order in
Choose your fights wisely -- some of them just aren't worth your energy
Be nice to everyone -- you don't want to give anyone a reason to hate you
Anything you say can and will be used against you in social warfare
Be yourself -- unless being yourself means doing weird dances in front of your chorus class. That, you just gotta tame or something. People will think you're insane.
And finally, the most important thing to remember in high school: stand by your friends. They'll always be there for you. And if they're not, they really weren't your friend to begin with.