P.W.
Sounds like a normal mom reaction to seeing her little boy all beaten up.
Do they box with gloves? That sounds like an extreme amount of damage from a little boxing. I would ask the school what the normal boxing protocol is. Don't get all angry with the school, it would be too bad if all boxing is banned because of one mother. As long as it is done with normal boxing regulations, it should be allowed, if it's a school sport. Sometimes parents make things so safe that kids are never allowed to do anything any more.
Meantime, beating each other up (in a loving way) is what boys do. My son and his friends used to throw each other all over the place on the lawn out front of the school. My son still has a sore knee from that.