My daughter has had a few (Y7) mostly for messing around in class, or being rude to teachers. I got called in a couple of times and fully supported the teacher and the next day, took her phone away for a week.
Last year, some older boys - one in particular targeted her with racist bullying. School didn't do anything, as nobody had seen anything. He used to corner her somewhere quiet and kicked her a few times.
A bruise wasn't enough, as no witnesses, so they treated it as he said / she said.
He got her one day and she punched him.in the mouth, he fell down half a dozen steps, banged his head, bloody lip etc. I had to go in and they wanted to put her on report (get a card filled every lesson for a week). They wanted her to apologise.
I told them that I didn't support the detention, but she would do it. I said that she wouldn't be apologising to a racist bully and that if they gave her a report card, I'd rip it up, rather than sign it.
I agreed that punching him was technically wrong and pointed out that he'd been kicking her and it was self defence.
I suppose that, because I'd supported the school previously, they accepted that I take her behaviour seriously, so they agreed to just a detention.