You did miss something earlier in the thread, it may have come from a teacher. It goes along the line that the skirt is of the required length when girl leaves home but is then shortened in a way girls know how, I think it's by rolling over from the top.
Must admit that some of the skirts worn by youngsters on my bus are enough to make me blush.
I love a bit of leg but this is just wrong with the age of these kids. Trousers seem a sensible choice.