My son is 4 and has never bit or attempted to bite anyone. Then this week the teacher brings him out to me and tells me he left a mark on another student by biting him. He acted like it was no big deal and he was justified in doing so 'becuase I wanted to use the carwash and he wouldn't let me". They will kick him out of preschool (which I don't completley understand either) if he does it again. How do I prevent this from happening again because our conversation didn't seem to get the message through to him?