encourage the kid, because hes resorting to, "violence" because that is what he is around. also, try to get a counselor involved.
well, the teacher should not yell and get angry. so 1, keep emotions in check. and 2, the teacher has to find out whats going on because it may seem like harrasment, but there might be a backstory.
some activities to help the kid is definitely getting him in a better environment, and around other students.