Do you think what she did was wrong? What would you have done?
I've already said I don't think she was wrong. And given an example of when I did something similar. Which, incidentally, I expected to be punished for, but wasn't because no one reported it to a teacher.
The problem is that this boy has clearly complained, and possibly his parents too, and schools have a set of guidelines they have to follow. And someone will have argued that feeling threatened doesn't justify a violent reaction. It doesn't matter that I think she was right, that the head teacher might think she was right, they're going to have to do something because it's been raised. In the states it's even easier to sue a school, and a principle will be terrified of that.
As I said, the system needs to be entirely overhauled, because the current situation is not okay.
But even within the current framework, the punishment is disproportionate and the school could have dealt with it in many less severe ways. The fact they didn't makes it even more disgraceful.