Why are the girls at my school pushing the school to make it compulsory to wear spandex shorts under their school uniform skirts?

An overwhelming majority of girls at my school are trying to get the school to add to the school uniform policy that it should be compulsory to wear black or navy spandex shorts under the girls school uniform skirts. Why are they doing this?
Why are the girls at my school pushing the school to make it compulsory to wear spandex shorts under their school uniform skirts?
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