Oakenbeach
Schools are increasingly developing absolute rules, in my experience. So "if you do this then this will happen". I think it's so that there is fairness and transparency. No one is treated more or less favourably.
To use your example hitting for any reason isn't tolerated. No such thing as they deserved it.
Behaviour does seem worse now than when I was at school for example. Schools seem to be dealing with issues that weren't around then and some behaviours are more extreme.
I've been a school governor. We've excluded students for bringing knives or drugs into school, for fighting in or out of school, for serious bullying, for attacking staff, for behaviour so extreme that it's prompted the class needing to be evacuated for their safety.
I can understand why they are having to be much stricter now with behaviour management and I am including the boy in this.
On the face of it I can't understand why his punishment isn't at least as severe which is making me think there are factors which the OP is aware of.
I think in the first instance the OP should speak to the school, especially about the events leading up to this. Just don't minimise what your daughter said.