I understand the maximum possible penalty to be quite harsh - that is, the violation of 1-3-113 prohibiting purchase, possession, transportion or consumption of alcohol by persons under the age of 21 appears to be a Class A misdemeanor. Class A misdemeanors carry a fine up to $2,500.00, or confinement up to 11 months, 29 days, or both. (I would venture that your likely penalty would be probation and a fine, although your possible penalty would be a year less a day in jail with a fine.)

You should consult a lawyer to see if there is a way you can avoid getting a criminal conviction, perhaps through a delayed disposition or diversion program (if available).