Could you specify a bit further?
If you're an athlete, did you sign any sort of contract? Many school districts have a contract required that not only sets up the usual medical mumbo-jumbo, but sets up rules for other things too. "Strike" policies against drinking, drug use, etc are often covered under these contracts, along with sportmanship clauses. If you signed a contract, the district can do such things against you - it's in the ink. This is not to say that such contracts are legal, but any contract would be binding until found otherwise.
Now, if you're a student, the story is a different issue. You obviously don't have a contract, which limits what they can do. You could be banned from going to events, but under most cases, only if they believe you're a threat, which requires that you've done something in the past, which in turn usually means that you did it on-campus. If this is the case, follow the advice of the others: try to talk it out with the school early, but have a lawyer or ACLU waiting. The odds are this policy is not legal, but few school districts give up fights easily.
Edit: And no, IANAL, but I was part of a student group that looked at re-designing our own district's "strike" policy