I've had experience at the parent of the other child.
Having moved form to get away from the bully who was telling all the others to exclude dd, we had a fairly quiet few months. Unfortunately they then mixed up one of the groups and put in together again.
Dd managed to avoid her for a bit, then she started saying loudly that dd was "incompetent due to her disability". It started as whispers, then she started saying it loudly in front of everyone. And to dd's surprise actually this time people stood up for dd and the group went to the head of year about it.
I then got a message from the head of year to say the bully "feels that she had been totally misinterpreted and it was upsetting her that people were judging her on it." and suggested we met as parents and children to discuss it.
I was severely unimpressed and pointed out several things. Firstly, as he had witnesses to what was said, how could it possibly have been interpreted as anything that wasn't unpleasant. Secondly, when she was telling lies (and I have witnesses that said she was) about dd the previous year she wasn't too worried about people judging dd, so why should dd have to sympathise with that when the bully had convicted herself with her own mouth.
And thirdly I was not prepared to meet and put dd in the situation where she would have to relive it again.
Further investigation went into the situation, which was easy as there were plenty of witnesses, including teachers. Then the bully was asked to write a letter of apology, and the mum refused to let her and kept her off school to avoid it because she was "so upset at having to apologise". Which was good in the long run because it meant that it became immediately clear that she was unreasonable because if it had been a genuine misunderstanding then the natural reaction would be to apologise and finish it.
Nasty child, nasty parents clearly.
Now my point is that the actual situation had been going on for a long time, and things had been said for well over a year. But it focussed on one because there were plenty of witnesses and clearly unacceptable.
I'm sure the parents were saying "it's just one remark" and "she didn't mean anything by it". I think the phrase "there isn't malicious bone in her" came out too. But it had been an ongoing campaign in which dd had moved away and compromised to avoid it several times and this was just the first time she was really called out on it.