Well, I probably won't be the most popular person on this thread, but... what is wrong with you people?
This girl deserved a broken nose because she teased someone about a sports team?

I can't believe grown adults would think this.
Hitting first is not the right thing. Hitting a girl is not the right thing. Telling a teacher or parent
would be the right thing. And schools today are much tougher on bullies than ever before. Many schools have a zero tolerance policy for bullying.
This may be because I am old fashioned, but I always tell my son to avoid physical conflict whenever possible, and girls especially are off limits. You see, I want my son to understand that if his wife or girlfriend were to get in his face and scream at him, it is not okay to hit her. Law enforcement and society do not look kindly on the "wife beater" mentality.
You may not agree with the way the OP handled the situation, but your shock, and calling her decision to ground him "ridiculous" is ridiculous. She made a decision that she felt was best for her child. She is probably trying to raise a civilized child.
With that kind of advice, I hope you won't be surprised when your kids end up in a juvenile detention center for "taking no prisoners."
Huh? I don't think my kid is "brainwashed" because he has been told by the school, and by me, that he is to inform a teacher, adult or me if he is being bullied. In fact, I would consider that "sticking up for himself".
So completely wrong in every possible way. We do not live in a vigilante society where we attack others. We have laws(code of conduct - for schools) that if followed, keep us relatively safe. A bully does not "attack more" if their behavior is relayed to a responsible adult. In fact, the opposite generally occurs. And others do not join in with the bully and become bullies themselves because of a student doing the responsible thing. For me, the above comment was the most bizarre and uninformed comment of this entire thread.
To the OP - the way you handled this situation is commendable. Your children are lucky to have you as a mother. There is nothing wimpy about teaching our children how to deal with bullying behavior in a civilized manner. Your children will be the better because of your excellent parenting.
As to the bully - I used to work in a school setting. I also have a child who has been on the receiving end of a few isolated bullying incidents. In my experience, I have found almost always that bullies are scared (and sometimes abused or neglected) kids. There is a reason they behave the way they do. It is because they have extremely low self esteem. I know many on this thread will find me wimpy or "living in la-la land" for saying this, but often times showing the bully kindness or listening to them goes a very long way in curbing their behavior. I speak from personal experience.