Lots of school systems require uniforms now. Ours does not, but schools may adopt an optional uniform. The magnet school dd will attend for kindy next year has had an optional, and mostly ignored uniform for years and we just found out that they are actually putting the pressure on for kids to wear it this year. In our school system the uniform can only be mandatory if there is a program in place to purchase uniforms for low income families.
I am really not excited about the uniform. Dd is a kid who expresses herself through her clothes, but she is happy to shop at thrift stores and get hand-me-downs. We are spending more money on uniform clothes than we would have on regular school stuff, and it is ugly! Her school is just requiring navy, white, or gold tops and navy or khaki bottoms, so it is flexible...but boring. Why are uniform colors never pink or purple! So sexist!
I'd rather just have a required uniform jumper really.
Oh well...anyway, to answer your original question, I would first check the regs for your school system. If they are ok with uniforms, or at least optional uniforms, I would check with the PTO.