Its both a school issue and a criminal issue. Knowing how schools work, it was most likely discussed with the School Resource Office (if they have one). The SRO can then make a decision on moving forward with the issue. If it was only a rumor of the kid drinking, the SRO is most likely not going to issue a citation as he needs proof that will hold up in court.
If the kid is an athlete, he most likely signed a code of conduct with the school. This is a contractual relationship with the school and not a criminal law relationship. Contractual relationships can be enforced outside the law and for some reason the school choose to do that. We do not know what the schools investigation has found. Maybe its not his first offense. Maybe he admitted to it behind closed doors. Maybe none of that happened and he is innocence. We do not know all the details and anyone outside of the student, his family and the school administrator that dealt with the problem does not know either.
In most cases the school will not discuss the issue with anyone other then the student and his family. This includes the kid making the accusation, other students or teachers. There are legal reasons for this. He is a minor and in most states the school will legal consequences as well as put itself in a position to be sued.