I'd have a bit more sympathy with the 'what does it matter how short their skirts are' attitude if we were talking about young primary school kids who are often blissfully and innocently unaware of this kind of thing.
However, most teenage girls are not at all unaware and are choosing to wear ridiculously short skirts. Even when they have a normal length skirt, many roll them up. Why do they do this? A combination of peer pressure, wanting to look like the cooler girls, attracting the boys they fancy, pissing off the teachers, rebelling against parents. Well I'm sure we all remember what it's like to be a teenage girl.
Skirt length has been a problem in schools since time immemorial, and it's tedious, a bloody waste of school time and a potential minefield for staff. I don't see any downside at all to insisting on trousers. It kills two birds with one stone - 1) avoids inequality of vlothing choice between boys and girls and 2) stops the skirt length problems.
Incidentally, although I agree that not having a uniform seems to work well in other countries, I can't see how having it here would get rid of having to have annoying clothing rules which are hard to implement. 6th forms seem to have exactly the same issues with skirt length when they aren't in uniform, with additional complications over ripped jeans, no jeans, shorts, strappy tops, flipflops etc.