Maybe, but banning him from future games suggests that would not have been the primary reason. The first amendment is under attack. Long live the 2nd.
Nope. It was completely harmless and all in fun. The school official was in the wrong, all the way.
If some attendees at a game react to a sign they don't like by committing assault, the proper reponse is to have THOSE people arrested and criminally charged -- not to have a government agency impose a penalty on a citizen who did nothing wrong and had a First Amendment right to make whatever statement was on the sign.