Typically when schools decide to allow girls to wear trousers they forbid them from wearing skirts. If long triousers are ruled in, shiorts are ruled out. Or the other way round.

The basic rule in these matters tends to be that of the ants in T H White's Sword in the Stone - "Everything not forbidden is compulsory", with its corollary, "Everything not compulsory is forbidden". Of course sometimes the compulsory stuff is social and peer pressure rather than formal rules, but it comes down to the same thing in the end.

The actual stuff that is forbidden or compulsory can vary - perhaps the rule is cover up, perhaps its the reverse, perhaps it "be formal", perhaps its "be informal". But underlying these superficial differences, the same rule applies - "fall in line".

And all this applies in society outside school just as much.