The fact that they are "boys" doesn't absolve them from acting appropriately during recess.. They may think they are only "playing rough", but what happens when their rough play injures another child? If you had a daughter in the first grade and she was injured by a little boy that was "only being a boy", wouldn't that upset you?
You have to make it perfectly clear to them that their behavior is unacceptable.. Perhaps on the days that they are sent to the principals office for not playing properly during recess, you should follow up with not allowing them to play at home after school on those days..
It's really not fair to the other kids on the playground.. And I don't understand why you are angry with the school about this..