Full disclosure, I say this not as a parent, but as a student of psychology and child development.
Like others have said, it is pretty normal for someone that age to experiment with things like alcohol and marijuana. I think it was smart of you to not over-react. Encourage a frank discussion on drugs and alcohol, why she does it, does she know dangers, does she know how to safely do it. A huge part of someone's choice to participant in drugs is peer influence. I would pay attention to who and how much she hangs out with certain people. That doesn't necessarily mean restricting her from certain people, unless they show continuous negative influence. I would also set very clear lines of consequences if the behavior gets worse, be clear, consistent, and follow though. If grades start dropping, then x. If you fail a test, then x. If chores don't get done, then x. If you ever drive under the influence, then HUGE x. I'm not advocating marijuana's use, but simply telling her 'no' probably won't be successful.
I really hesitated to give my .02 since I am not a parent, so take my opinion with a grain of salt. Never let her forget she is cared for and safe at home.