Living in Scotland and being surrounded by men and boys who wear a kilt as a matter of course (traditionally without underwear), this whole argument seems utterly bizarre.
Private schools in Edinburgh and elsewhere have kilts as part of the school uniform (public schools generally don't as this is a more expensive option than trousers).
My boys just got fitted for a kilt yesterday and they wear it with pride. None of us have ever considered the garment as impractical. And they don't flash anyone either, despite the lack of underwear (it's all in the design).
And skirts in general are not either - some versions are indeed impractical as are some versions of trousers. That's not a reason to ban the garment altogether. It's 2019 for goodness sake - offer all choices up to all pupils, establish rules as to skirt/trouser/shorts length and type and then enforce the rules around that. And then there's no need to involve the police in penalising girls for wearing skirts.
Whatever anyone's personal preference is in this regard, skirts continue to be a popular choice among girls. I don't see anything remotely progressive in banning them.