You're not describing a boy who has an aggression problem. You're describing a boy. This situation is representative of the tendency on the part of schools to overreact to aggressive behavior of any kind. Because boys are generally more aggressive than girls, boys are the usual targets of these overreactions.
Schools — public and private — seem to be having great difficulty differentiating between what is simply normal boy behavior and what is truly pre-sociopathic behavior. They end up punishing boys for simply being boys.
The more immediate problem, however, is the practical one: to wit, most private schools — especially those with waiting lists — have no reservations about expelling problem students. As one student goes out the door, another student comes in. I don't need to tell you that if your son is expelled, it will be difficult to find another private school to take him.
There's another possible dimension to this as well. I'll just bet your son is not only having fun goofing around physically with other boys; he's also having fun out of getting such a disproportionate reaction from so many adults. Unfortunately, all of this is likely to lead straight to a one-way ticket through the school's front door.
Level with your son. Sit down and tell him you understand he's having fun as opposed to being bad, but if he doesn't stop, the school is going to kick him out. Furthermore, tell him that, as much as you don't want to, you're going to have to punish him when he goofs around. That's the nature of your agreement with the school.
In that regard, whatever punishment you use is going to have to more than cancel the fun he's having. When the next incident occurs, take away all privileges and put him to bed early for two weeks. Whatever you do, it's going to have to make a permanent impression.
Family psychologist John Rosemond answers parents’ questions at www.rosemond.com.