I'd probably take a different tactic.
I'd ask the principal of the school to give me a plan about what they were going to do to keep the bullying child away from mine.
It would probably go something like this:
"We have discussed with Johnny the fact that he cannot defend himself from a physical attack from another child, because he will get into trouble too. He does understand that.
Because the child that attacked Johnny has a known history of physically aggressive, bullying behavior, we would like to see a copy of the school's plan as to how they are going to handle the child. We are specifically interested in learning how the school intends to monitor that child's behavior and keep him away from our child so that our child will not be pinched until he is bruised and otherwise physically attacked.
We are putting the responsibility for our son's safety at school directly onto you, Mr. (or Mrs.) Principal. We will await your response as to how you intend to handle this situation so that our child is not in a hostile learning environment."
Toss that out there...see what happens.