I am shocked at how my sons Elementary school disciplined him any suggestions on how I should move forward...

I have an 8 yr old little boy in 3rd grade in a public school in Gilbert,AZ. This is his 3rd year in this school. On Friday I was called and told that he had pushed a 1st grader out on the playground and the child had been injured. That was basically all I was told. I found it odd that I was called and informed of another childs injuries but, I did ask if the child was ok and left it at that. When I picked up my son he said that they were playing and that when he pushed the boy the boy was wearing sandals and because the ground was gravel and cement he had slipped therefore falling. I asked if he had told the teacher that and he said that they wouldn't let him talk. he said that they told him that when playing with little kids he needs to be more careful.
I told him that I agreed that he should play with only big kids from his class.
This was Labor Day weekend so he didn't return to school until Tuesday.
When I picked him up from school he was in tears. I asked him what had happened and he expressed that he had been put in a little room all day.
I went in the next day and asked to see the "little room" that my son was put in the day before and the office staff lokked at me like I was nuts. They finally asked me to sit down and after a few minutes of talking amongst eachother (the principal,asst.principal and office staff) they allowed me to walk around the corner to see the 3 doors that looked like broom closets w/ small windows. They had small desks in them and when a child is in the desk there is no way they could stand up to stretch for a minute. Apparently my son was in there from 9:30 a.m to 3:40 p.m. even having lunch in there.
Sorry but I feel like that is terrible. I would not put my son in a broom closet at home and I surely would not allow someone else to either! But, I don't know what to do. I am scared if I put in a complaint to the superintendent then the principal will treat my son terrible and he is doing so well in school now. He loves his teacher.
Even other teachers have made mention that they feel that the principal uses this closet treatment when the principal doesn't want to put effort into the kids herself...
ANY IDEAS!?!?!?