In public schools, uniform policies are a useful tool to simplify the management of behavior. If you have a lot of students dressing in ways that are sexualized, distracting, offensive, or that in some other way take away from the educational process, you have two choices: either spend a lot of time and manpower dealing with individuals on a case-by-case basis and making dozens of judgement calls, or else just institute a uniform policy that is quite clear, and then punish those who don't conform. Uniforms save a lot of work for the administration, and that makes them a tool that responsible leadership teams should consider.
I personally don't like school uniforms (I wore one for three grades), but I also don't like having to constantly tell boys to pull up their pants, girls to pull their necklines up and their skirts down, and having to read every tee shirt that I see all day to check it for references to drugs, alcohol, or offensive content. I'm a teacher, and I would like to teach. That's all. Students who insist on wearing offensive attire in the name of "self-expression" take away from my work, and consequently from the education of every other student in the room.
Maybe someday tweens and teens will figure this out and just put on clothing that isn't intended to start something. Until then, the uniform debate will be alive and well.