I have been amazed at the change in my son's behavior as he's entered preschool (School Readiness). He's 4 1/2 and has always been very physical. He bit/hit until a little later than most kids, but was in a very loving, touchy daycare who worked hard with him to teach him not to do those things--they removed him entirely from the situation and had him do puzzles (he's very, very good at puzzles now!) and it stopped. Now that he's in school, I'm not seeing the aggression, but I am seeing his language and tone of voice becoming disrespectful. Is there the possibility you could observe for an hour, see how he interacts? It might give you a clue to what's happening. Maybe he feels he doesn't have control, doesn't know what to do in a situation, or is even being taunted by other kids, all of which you could help teach him to deal with in better ways. Perhaps he's just aggressive, in which case it's a harder lesson; maybe his teacher has advice? You seem to be very accepting and ready to work on it--most teachers are very excited to work with parents like that, especially if the student is truly having troubles. Also, without diminishing the need to teach him that this is not acceptable behavior, I do think it's somewhat a boy thing. We showed affection by (ok, it's weird to write this out...) kinda kissing like dogs a little, I guess? Roughly? We'd be roughhousing and using our mouths and not biting, but using our lips to tickle (this is not perverted, I swear) and later wondered why our 1 year old bit...gee...we used our mouths for affection; he probably just didn't quite know yet how to control his teeth! Look at your own behavior, too, and see if there are subtle ways you're "encouraging" it--I think kids need rough play, but I also wondered if he couldn't tell the difference between roughhousing and aggression. Anyway--good luck.