If a group of girls had barred this girl's way out of the toilets, and she had thumped one of them, what would the school have done? Would they have punished them all? Would they have punished the victim more than the bullies?
I can give you a concrete example of this one as I used to teach in a girls' school.
The girl who had thumped one would have had a punishment. Depending on damage done that might be a telling off, a detention or an exclusion. I've seen first and last options. First option for pushing a girl out of the way and against a wall, third option (internal exclusion) for a hard slap that left a bleeding fingernail track.
The bullies in these incidents were also punished. First incident, with a massive, massive telling off, put on behaviour report, and a pointed Head of Year conversation with the whole class that left them squirming (year 7 age).
Second occasion, one bully 2 day internal exclusion, plus detention, plus behaviour report, second bully detention and behaviour report. Both told any further incidents would lead to proper fixed term exclusion. Year 10.
I've only ever seen one student expelled. She brought a knife to school to "show them". Not "to show them" in the menacing way, just "I thought they might like to see my knife" according to her. The whole year group got a massive bollocking because lots of them saw it and didn't report it.