@-@ She's fourteenish...she doesn't need beer/drink/drugs/cigs..! (Please tell me you're a girl..I'm awful with genders...)
To start, it all kinda depends on the HS. :/ I'll give you advice on what I gave my brother (he'll be a Freshman) and hope it works. I'll be a Junior, btw.
1) Don't do drugs. -.- Seriously, they get addicting after a while and then you can end up as a drug addict and get a bad reputation. Your grades may go down and you really shouldn't ruin your High School career by doing drugs. HS is pry going to be the best/most fun time of your life. Really wish to ruin it with alcohol and such?
2) The upperclassmen aren't bad. They aren't really, not where I live. Pretty much you have to know who to stay away from. Mainly because there will be those jerks who will get angry if you step on their wrong side. But so long as you're a good kid and don't pick fights with them you're golden.
3) Study. Seriously, I hate studying, and so therefore I don't..I'm a bad example, my test scores aren't the greatest, I could get better. But do study, it does help.
4) Do your homework and such. Your homework may end up being more of a percentage on your final grade than some other things. For one of my classes I believe it was 40%...yeah, it makes your life oh so much easier to do your homework.
5) Meet new people, your class and peers will be graduating with you..getting on bad sides really can't be a god thing. -.- Make new friends and if someone does pick on you, ignore them, they're not worth your time and are rather immature for picking on someone. ^^;
6) Have the time of your life. Enjoy and it don't throw it away, do what you want, just don't get into trouble. Meh, it's your life and you should use it the way you want, and advice never hurts.
and 7) Don't argue with the teachers, they hold the detention slips.