My 6 year old son is misbehaving at school I cant get thru to him that his behavior is not ok what do I do?

My son is 6 years old and I am getting a phone call from the princepal almost every other day because my son has done something to another child. He has been suspended from school 3 times now because of his actions towards other students. What worries me the most about his actions is that when I ask him why he did it he tells me he didnt do it, or that he doesnt know why he did it. The teachers see him do things and he tells them I didnt to it.. And the worst part of it all is he hurts others and shows absolutly no emotions. Even when he gets in trouble he just stares right thru you. and I dont know what to do. I have had him on medications for a.d.d but they did nothing for him. any help at this point would be great thank you...