Someone recently asked me what I would, if given the opportunity, tell my 14-year-old self. My first thought was that my 14-year-old-self would never listen to what I had to say - my 14-year-old self would have absolutely no interest in any advice herself from the future would have to offer.
But here goes.
Don't date the same person more than once. If it didn't work out the first time, it is unlikely to work out the second, third, or fourth time.
Date whomever you have a connection with, if that person treats you well. Make the decision to date a person based on what you think, not what others might think (or what you think others might think).
Believe in yourself. Have confidence. But don't be surprised if you fake it more than you mean it. Don't worry about what others are thinking or saying about you. If you knew how seldom they thought about you (or how little it mattered), you wouldn't care.
Try to avoid doing anything that you will truly regret in hindsight. Not necessarily getting into mischief (you should get into some mild mischief), but more to do with being less than kind to someone. Maybe you made fun of them behind their back, or worse, to their face. Or maybe you simply ignored them or judged them in some way. Avoid that stuff. You won't like yourself later - and neither will other people.
Don't eat cafeteria crap. It's bad for you, and you have no idea how many gross hairs or skin cells have found their way into your fries.
Make memories. Take pictures and cause a minor ruckus. Skip class occasionally and have brunch with your friends. Go to dances. Participate. Come up with new ideas and implement them. Play cards in the hallway during your spare period. Memorize at least 20 lines of your favourite Shakespearean play. Go on a ski trip. Try everything at least once.
Study. Even if you can still get 90s without it. Someday you'll wish you had studied more. Do your best, but do it on your terms. Don't push yourself too hard, and don't let others push you. Achieve things, but achieve them at a pace that works for you. Set your own goals and be your own reward.
Buy a yearbook every year and get everyone in school to sign it.
Trust your teachers. Most of them. Some won't be perfect, but some will be there to help you. Remind them what it was like to be a student. Be reliable and hard-working, and when you need a break they'll be happy to provide it.
Finally, at my university graduation ceremony I heard something that made a lot of sense. It's a famous quote by Winston Churchill, that has since been misquoted so many times that it's probably been taken totally out of context. ...But the story goes that Churchill went to a school to give a speech. He stood up, looked at the students and said: "Never, never, never, give up. Never give up." and then sat down. It doesn't matter if the story isn't quite true, the sentiment remains the same.