Fred:
You have the right to tell this person, one-on-one, in private, that you are worried about her. That's what friends do for friends. If you decide to have that conversation, don't get into a whole lot of "you're ruining your life, breaking the law, you need help", etc. Just a simple statement that you're worried, and would like to be there for her when she's up for doing stuff that doesn't involve drugs, booze, etc. You can invite her to things like that from time to time, as well.
Outing her to school authorities could really, really backfire on you. I double-dog promise you that it will somehow come out that you were the one who told on her, and your life will become a living hell. Depending on how strong the drug culture is in your school, you could actually be in for some physical danger - it wouldn't be the first time a "snitch" got a beating to "teach 'em a lesson." I know your school probably has an informant programme, and that they say everything is confidential, but there's the way things are supposed to work, and then there's the way they really work. Don't risk it. Drugs are Big Money, and by informing, you could be interfering with someone's profits. That is dangerous. This is real life, not a movie, where everything turns out okay.
If I were you, I wouldn't make a big thing out of AA or treatment stuff right now. As others have pointed out, a lot of experimentation goes on at your age. Most people come through it okay, more or less. If she needs intervention, it will become clear to her family, who are the proper people to do it.
Just be there for her on her good days, and hope for the best. Preaching to her will turn her against you, and she'll probably tell her other friends that you aren't "cool", which will cause you problems. Go your own way, and invite her along for the ride, but if she doesn't accept, then you have to find other friends. No recovering addict or alcoholic ever got truly sober and well because of other people's concern, intervention, or efforts. They get well because they come to believe that things have gotten so bad that they have no choice but to accept that they're powerless over their drug of choice, and that their lives have become unmanageable.
Best of luck to you, and to your friend. Being your age isn't nearly as easy as it was when I was there.