I'd buy her two new sets of uniforms. Ask the school if they could arrange her a locker / if there's a safe place in the teachers break room where they could keep her uniform.
On Monday week 1 - take SD to school with the first uniform. Get changed at school in the morning before the day starts, leave her regular cloothes with the teacher. Ask teachers to make sure SD changes back to her regular clothes before mum picks her up. Uniform stays at school.
Monday week 2 - take SD to school with her second uniform. Repeat what you did week 1, but also take back the uniform she wore week 1 so that you can get it washed.
Repeat.
If this doesn't work, I'd send mum a text asking her to pack SD a uniform to bring with her on the non-school day. In that text, make sure you record that you're not asking mum to provide a uniform she's paid for, but you're just asking her to return a set you have purchased. If she refuses, try get the court order modified. The above text you send her, and her reaponse need to be filed to court as evidence of mother being unreasonable (so make sure that the text you send her is very polite). You can explain in your court application that due to mother's behaviour, your options currently are:
1.Pay x amount of money per year for having to purchase SD a new uniform every week. This means SD suffers, as you have less money to do fun things with her (holidays, hobbies, family days out, etc)
- Send SD to school in regular clothes. This also means SD suffers, as she finds this embarrassing.
- Juggle with the school doing the above uniform rotation. This also means SD suffers as she needs to wake up earlier to go to school, and it's embarrassing as her friends wonder why she needs to do that
Best of luck with it, you sound like a great stepmom and it's good to hear SD has at least one positive maternal figure in her life. The bio mum sounds like she's a narcissist.