Interesting topic and may be going into the history of things would be important. Tribal villages from different parts of the world do show a tendency of unisex dressing - wrapping something around private parts and same for men and women.

Unless one know whether women chose to dress differently or whether men forced them to dreas differently, its difficult to say whether the idea has always been sexist. It may be the case that during time when man was hunter gatherer and was the breadwinner, women chose to dress differently to appeal to the opposite sex.

Times have obviously changed, so women now do have the choice to dress the way they want but i dont think the skirt is sexist. Having different styles of clothing also helps increase consumerism and clever marketing can be used to make millions.

Lastly, schools should have gender neutral dressing and should not have the power to dictate what kids should wear. It can be a thing parents decide till kids can think and choose what they want to wear.