Angry 6 yr old in class?

Ever since his teacher came back from maternity leave, my son has been having fits in class like throwing books, tipping over desk and chairs, and kicking the teacher. He likes his teacher but loves his substitute and was angry when the sub left. That was back in feb. We are also in the military and about to move. He is upset with that too. This also happen before in day care when his provider left for surgery and came back two months later. He went nuts. I had to pull him out and he calmed down. What I don't understand is he changed schools and location and houses with no apparent problem but when an authority figure leaves momentarily he can't handle it. I don't know how I'm going to get thru til next month. Pulling him out now is not an option and it doesn't teach him to deal with his anxiety. He was fine in pre k and kindergarten so I dunno since feb he is impossible for the school to deal with. Any ideas??