How should I have reacted to this? ?

I would want to be sure that he had told the true version of events and that he wasn't actually being bullied. Once it was clear that it was true about the girl kneeing him, I'd speak to him calmly but seriously and ask him why he acted that way towards the girl and make it clear that I didn't think its ok to act like that. You want to be authoritative without sounding aggressive and allow your son to explain himself. Don't be too heavy on him as lots of teenage boys act like that, usually to try and impress other boys at school.