Wow, that hasn't ever happened in any of my classes. *Hahahahaha!!!!*
Actually, I've come to pretty much expect at least one problem like
that per class per term. Kids have to do something like that to test
your mettle, and see if they can make you mad/cry/quit your job,
I've had the most success with situations like this by avoiding
punishing the entire class. When you do that, it's a badge of honor
for the kid who committed the crime, and it hurts the ones who had
nothing to do with it and who really love art class projects.
Most of the time, you can pull one or two kids out at a time, (usually
the ones you THINK probably did it,) and if you have a good poker
face, you can act like someone's already told you it was them.
Usually they'll spill everything they know within seconds, even though
this is the oldest cop trick around. If not, try it on another kid.
It's all about mind games. *sigh* You just gotta love kids. :O)
Good luck with the administrator who wants you to quit painting, too
-- sounds like they're not much above their level, unfortunately. I
hope they wise up.