Kids have to have clothes everywhere in the world, and kids outgrow them quickly everywhere in the world. If kids did not have to wear uniforms, they would have to wear something, and whatever that something was, it would not be free.
Wearing a uniform influences the student to behave and perform like a student.
Teaches dressing "formally" influences the students to respect and obey the teacher.
Everyone dressing in play clothes influences everyone to play.
The education my boy received, and atmosphere he was educated in, was far superior to the education he would have received than he would have received at a typical public school in California where I grew up.
It's not the poor that do not want the kids to wear uniforms. It's the rich that do not want the unforms, and they are pretending to care about the poor to make the case.