In what way? Poor children wouldn’t afford the same attire as rich people, no matter with or without uniform. You can still have more expensive and therefore bully those that don’t have the expensive ones. You can still bully a kid for having a cheap Android phone and not the latest Big McChicken Pro Max™ iPhone; you get the gist.
Uniforms just hide the issues of classism and never deal with them directly or even attempt to solve them at all.