That's kind of a broad question. It depends on how much you get caught with, if it's your first offense, if your principal is a douche, etc.
First offense is a 3rd level misdemeanor in PA; max you'd stay is 30 days and pay $500 for simple possession (under 30g in PA). You'd get kicked out of school.
i heard this guy got caught with 2 joints, and he got 10 years. but thats hear say. it depends on your area, how much you have, who catches you. etc etc. some cops will just take your bud, and tell you to go home. but it all depends.
[ame=http://www.youtube.com/watch?v=Mq0DANKfsvM&feature=related]YouTube - Hypnotist vs Police Officer[/ame] just learn hypnosis