If a child hits your child in the head with a basketball at school, would you expect that child's mother to reach out to you?

First of all, something very similar happened to me as a child, except I was attacked by five boys at once with a basketball. My parents immediately scheduled a meeting with the principal and the parents involved because situations like that should never be ignored. Although I did receive an apology, and gifts involved however strict measures were also put into place afterward. The students were instructed to stay completely away from me, and the school staff — including security and even the janitors — helped keep an eye on the situation to ensure my safety moving forward.
Personally, if I were in your position, I would absolutely make a police report and clearly explain how serious this situation is before something even worse happens. No child should ever have to experience that kind of treatment in the first place, and addressing it early can help protect your child and potentially prevent harm to someone else as well.

I'm sorry this happened to your daughter. Not right at all! Not that it's justifiable but I bet that child probably behaves that way and/or bullies their parents too and the mum might be embarrassed. Sounds like they might be a little beyond patental control. I wonder what's happening at home and why the school are allowing it/what they are doing to help resolve his behaviour.

To be honest, I’d expect that his mother is part of the problem. The way this child has behaved needs to be addressed in a firm but compassionate way by school staff and the situation needs to be monitored.