I can't stand this "both sides made mistakes" kind of wishy-washy crap. The teacher is responsible for the consequences of his harassment.
No, he's responsible for provoking her to the point of retaliation. She is responsible for the nature of that retaliation. By your logic, she could have shot him and that would have been his fault.
Although that said, I don't think bagging him was an overreaction. It's a bag, not a sledgehammer.