I don't think you are over reacting at all. Call or email the principal to find out exactly what they are supposed to do in this type of situation. Talk the the counselor too - it's understandable that you are still freaked out about this.
I'll tell you what happened to my daughter (2nd grader) on the bus ride home. Last Friday, a girl in her grade attacked her on the bus. The girl hit my daughter and pinned her to the seat. The bus driver stopped the bus and said knock it off, but otherwise did nothing. I called the principal immediately, but she had already left for the weekend. Monday was a holiday and I heard nothing (expected). Tuesday, I was actually in the school library volunteering, but didn't hear anything from the principal. I told the librarian what had happened and she went and did some investigating for me. She told me that the principal was in the process of interviewing my daughter, the girl who attacked her and another witness. The librarian was actually most helpful. She pulled the girl that attacked my daughter aside, told her how wrong it was to hit someone, regardless of why, pointed me out as the victim's mother and told the girl how upset I was. The girl apologized and promised not to do it again.
Wednesday of this week, I received a call from the principal. The principal told me that the girl hit my daughter because my daughter was "mean" to her. The principal said that both girls got a warning. If they get a 2nd warning, they'll be suspended from the bus. I asked her to explain. The principal said that the other girl had asked my daughter if she could talk to her, my daughter said no. So that's why the girl hit her. The principal said that my daughter's meanness didn't justify a hit, but that it essentially was my daughter's fault she got attacked. I told the principal that I disagreed with her. That if my daughter doesn't want to talk to someone or play with someone or be someone's friend, that she didn't have to and that she shouldn't be attacked for that.
So, I ended up telling my daughter that if this girl hits her again, she can punch her in the face because she'll be kicked off the bus anyway.
They punish the kids that don't do anything wrong and protect the perpertrators because these kids have "emotional problems".
Good luck to you.