I sure understood at the age of 12 not to do such things.
It really depends on the painting.
Most of us did, however you occaisionally run across the "class badass" who just has to act on his stupid thoughts. He knows right from and wrong but he just has to do it.
Sometimes kids and adults too, do something wrong even when they know it is wrong regardless of how they were raised.
So did we. First of all we were told not to touch things that didn't belong to us. Second of all we knew when we went to anything like that that it was something special. I'm sorry. This kid knew what he was doing.
I wonder at home if he puts his gum on things around his own house. Like family photos, prints or any kind of artwork his parents have. Wonder how he would like it if his parents or someone else put a wad of gum on a favorite photo, print, poster or whatever else he might deem important to him? It would be a different story then. I'm glad he was suspended or tossed out of the charter school. He was/is old enough to know what he did was wrong. Most people even kids know artwork in a museum is there for a reason. He may not like the piece or maybe he thought it was silly or stupid and had no meaning to him. That could be the case but...I doubt it. I think HE thought he was being cute and funny. He should be made to have some kind of job that so much money goes towards the museum to help in the repair. Granted, he could never make it all up to them but..maybe it would teach the little twit a lesson that once you make a decision to do something so stupid you will have to pay the consequence.