So this boy used to be OH's daughter's friend. So, presumably, he has a decent side, and doesn't target abuse at people, most of the time.
The school should organise some lessons, maybe boys and girls separately, to talk about the issues and the need to be sensitive. All the kids need more help with puberty and phse. I can't imagine what the boys' sessions would entail, but I hope they would probe male attitudes to female bodies. The results might be encouraging. It might be that stupid random insults don't denote attitudes which equal the belief that girls have smelly orifices which are somehow not as nice as male organs. We don't know what the young boy had in his mind when he spoke. We are just hearing one version.
The girls' phse session ought to include, imo, discussion of personal care and hygiene, requests for feedback to make sure there is enough time to change using proper facilities and products in school. At the same time, if I was the mother of a girl, I'd suggest wearing little shorts over pants to make sure sanitary wear isn't slipping out of place. I don't think very short skirts are sensible for the reason that they might expose underwear accidentally and decrease confidence that everything is well-covered and not about to show accidental and unavoidable staining. If a girl is well-prepared, she can be confident and know there is no basis to such insults as the one we're discussing. She is clean and fresh to satisfy her own optimum standards and she can ignore pathetic, supposedly amusing comments. Then she can get on with her life. That's all any of us can do, as teenagers or grown adults; as menstruating women, pregnant or older women who have become incontinent. Sort ourselves out, be confident, and ignore insults. This is better than focusing on closing down juvenile insults, because rude and foolish people will always exist.
Is the boy's comment worse than "Your trousers smell of piss"/ "Your jacket smells like shit"/ "You've got a wank stain on your shirt"? Why is his 'fanny' comment sexual abuse? He is twelve. He might not even connect his comment with menstruation or think that periods equal a different smell. It might be a stupid comment his older brother makes sometimes, that he copied for something to say. Or maybe a crude version of " Shut your mouth or a fly will go in", for when someone has their legs a bit apart.
Also, I don't get asking people out at age twelve. If that is going to lead to kissing or experimenting, then it's surely too young. Also, imagine the risk of rude comments if a girl actually kissed and cuddled with with one of these oafish boys. Isn't it best to avoid boyfriend/girlfriend at that age?
I also don't get chatting about the insult and finding out a lot of the boys thought the comment had been out of order. Isn't telling an adult the best course, but then staying silent and treating it just as a contemptible random , childish comment, not a barb which had hit it's mark.
I'd like to think schools can do more, but I don't think making a social drama of any kind of insult is a wise idea. It just leads to side-taking and a them and us attitude - and aggression and score-settling.