Back when I was in high school I don't know if they'd have accommodated me if I'd asked for it, but regardless I attended a boys school during 6th form (the sixth form was mixed, girls were rare, the lower years were boys only) and I decided to start wearing black jeans instead of the uniform skirt one day. Strangely enough nobody noticed or cared, and this was an English school (also a church school) where uniforms are usually something they care about. Nobody challenged me on it in my two years there.
The shirts/ties and blazers were the same for boys and girls so the only thing I changed was the skirt. They were stretch jeans so I guess they looked neater than other kinds. I'm honestly surprised nobody said a thing about it, not the other kids, nor the sixth form headmaster. I guess it blended in somehow. Maybe they all just assumed I had some special permission/reason to wear them, lol. A case of confidence validating itself?
