Trespassing

What kind of event? What kind of property? Public or private? Did he continue to stay after being asked to leave? More details, please.

If he just went there and stood around, then trespassing would be it, unless he resisted arrest. I believe that kind of trespass is a misdemeanor. If you want him to get into trouble, then have him take a swing at someone or smash something.

I think it would depend on what exactly he did on stage. If he was just wandering around, looking lost or confused or stoned, they would probably urge him out of public view, and try to figure out what was going on.
If he turned up in costume, and tried to perform in the show, shouting down the other participants, more likely he's promptly hauled off and less sympathy is shown.
If he just appears on stage and starts a harangue, and fights off attempts to remove him, yeah, cops.
* Also depends on who's available to stop him - is it a couple of teachers? (I would imagine even if a couple of beefy student athletes were around, no sane school official would let them do anything, for safety, if not legal, reasons.)
Does the school have security guards and are they available? I doubt that police be on-scene, unless there are any in the audience, so volunteers might haul the kid off to a quieter setting, while they wait for officers to attend.

Oh, and if he grabs the microphone out of some teenage girl's hands, I hope this time Taylor Swift grabs the mike back and whacks him in the head with it. Best grad ceremony ever!

Think backwards. What does your story need to have happen? He could be let go with a warning all the way to having to appear in court. An arrest for simple trespass isn't in the cards, but if he had a pocket knife and got charged with armed trespass you can make it worse, if the story requires.

Jeff