conflicting potty-training

our dilemma involves our special needs 6 yr old son who is getting conflicting potty training between home and school. let me start this by saying that last spring & summer break we had our son trained to where he was telling us when he had to go and he was for the most part, consistently using the toilet. the only time we put a diaper on him was when he was going to bed and when we were going out. but then once he got back into school he did a complete 360...refusing to use the bathroom in the classroom and from what the teacher was saying, was actually getting quite violent with his protests, hitting, kicking, spitting, biting, etc. but then he's coming home and going right back into telling us when he has to go. what the heck is going on?! so, his teacher has decided to stop trying to work with my son on potty training but then complains about him still being in diapers. i understand they are extremely limited in how they can discipline the children and i've given them ideas on how they CAN deal with my son's behaviors cuz it's what works at home. i've told them that my son is doing everything he's doing because he's figured out that those behaviors work on them. he's successfully managed to manipulate them by resorting to violence and sees that he's getting away with it. he is NOT showing all these violent behaviors at home in regards to potty training, in fact it's the opposite, he WANTS to go to the bathroom and will express the desire to go just so he can simply sit on the toilet! i'm starting to think that theres something wrong with the classroom environment that he's in...i'm not saying his teacher is bad or anything, but why else would my son do complete 360's like that? fight like crazy in school but then be so willing to do it here at home... what gives?