DD only wore skirts/dresses all through primary school. They had the option to wear trousers/shorts and several girls did.
She still managed to run around, climb, did gymanstics and at her CM after school she climbed trees, used a swing and trampoline.
She slowly got less active in school breaks the older she got, but I could see the same in the daughter of a friend in Germany, pre-teens don't do this apparently, regardless of wearing uniform or jeans and hoodie.
DD had two sporty hobbies, so I wasn't too concerned about this.
I find the secondary uniform more ridiculous. Even the most traditional London banking sector is not that formal as my DD is now in 6th form or during KS3 and KS4. I hardly ever see my male colleagues wearing ties.
Get rid of uniform and do a general dress code to avoid nearly nude in Summer (no bare shoulders or midriff for example and no shorts/skirts shorter than just above the knee) and they will still bring in good grades.