I said "The girls are simultaneously less able to be provocative, and better protected" if they wear trousers (meaning, as considered by the people who run schools).
But there could be more. There's all this trans stuff around now, so if the girls can wear skirts, transgirls have to be allowed to as well. And then the school gets calls from people complaining that there are girls in that school's uniform with their skirts rolled up to nothing, and also people complaining that boys are wearing skirts.
But as for "boys allowed to wear skirts", it's already happened in a few places. Once warm weather arrives and the boys are forced to stay in long trousers, some of them have followed the obvious path of dressing like the girls. There might have been a time when no self-respecting teenaged boy would do this, but with so much gender uncertainty around, it may seem easier. If schools find that an embarrassment, it's another reason to say "Make 'em all wear trousers and we'll be done with all this nonsense".