One practice that I've seen work is to ask the other students to help
with the 'problem child.' Give the other students a chance to explain
how this students behavior is disrupting the class as a whole. Throw
the whole issue out to the students to discuss. Let the students handle
the dialogue; you facilitate. I like to use "koosh" balls - whoever has
the koosh ball gets to talk, everyone else listens.
On the other student who may be having issues with art, school or home,
I would imagine that s/he is using art to vent emotions, which is
completely natural,...and safe, for him/her. Yes, document it all and
hand it over to the student's regular teacher. You are one of the
student's teachers, have the ability and capacity to note behavioral
problems and have the responsibility to report them.
I'm sure that we all appreciate your concern for him.
Perhaps you can sit down with him and ask him about his artwork. "It's
very interesting;...what is this saying here, and over here?" Let him
project verbally into the painting since he seems to be doing that