But, this whole thing is clearly just about skirt length and is completely understandable.
I've just checked the uniform policy of every secondary within 10 miles of my house, they all say 'Knee length skirts'. I see girls every day on the way to school with skirts barely covering their bums. The schools have set a clear guideline, if they challenge the kids they get flack from parents, they lose teaching hours, they end up in the press, they waste stupid amounts of teacher time telling the girls off.
Telling everyone to wear trousers removes that.
If they allow shorts, the situation will be that the girls are wearing skin tight hot pants.
My DDs primary, have a very open uniform policy, anyone can wear dresses, skirts, shorts or trousers, nothing needs to be branded. My DD chooses to wear trousers or knee length shorts every day because they are more comfy.
Just look at the protest photos, those girls are wearing too short skirts, they haven't been rolled up, so their parents have bought them too short skirts.
School is not a fashion show or a dating location, it's reasonable that the school should want the children to be dressed in a manner that allows them to fully participate in all activities, with freedom of movement and without risk of indecent exposure. They also don't want for the girls who are not comfortable having their bodies on display to be peer pressured into doing it.