I completely disagree with uniforms on every level.

1. Supporters of uniforms say that they reduce bullying with regards to fashion, but there is a great deal of evidence that says it just pushes that bullying underground. Instead of being bullied about something superficial like the shirt you are wearing, bullies just go to the tried and true body image shaming. Glass? Overweight? Acne? Too many freckles? Hair colour? Too tall? Too short? What school administrators see is a surface level reduction in bullying, followed by them patting themselves on the back and ignoring the problem completely.
2. Supporters of school uniforms say that they make everyone equal, but this could be farther from the truth. First of all, bullies just look for other reasons to ostracize their classmates, as noted above. More notable is the second problem though. See, school uniforms are usually very, VERY expensive, and families that are struggling more can’t afford to buy their child multiple uniforms. As such, when their uniform gets dirty, smelly, torn, worn out, or too small, the student is forced to live with it, as they cannot afford a new one. Meanwhile, those who have more can come to school in a crisp, clean uniform everyday. Think I’m making this up? I have students who outgrown their uniforms immediately, and spend the next 2 and a half years in what are now capris because they grew.
3. Supporters say that uniforms reduce distractions in class, yet many uniforms for girls are heavily sexualized. I saw a student 2 days ago straight up molesting a classmate (though I thing they are dating), and constantly flipped up the side of her skirt while she was seated to reveal her bare legs. If she was not forced to dress like this, I doubt this would have happened. Meanwhile, on the male side of things, everyone is busy trying to look as cool as possible, which just causes more distractions. Further, I cannot count how many minutes are wasted at the start of every single class as teachers chase students about their uniforms. Fix your ties, tuck in your shirt, tie your ribbon, fix your skirt… This is the very definition of a distraction, whereas not having uniforms would save hours a year of harping on students… hours that could be used to teach them.
4. Uniforms usually reinforce centuries old gender roles, and are becoming more and more of a problem for LGBTQ students. Boys where normal clothes while girls are forced to wear skirts and blouses. That was fine 100 years ago, but we have more and more students no longer conforming to gender roles and sexual identity. Being a teenager is hard enough without having to come to terms with gender/sex non-conformity, all while having that gender conformity forced down their throats.
5. Supports of uniforms also say that uniforms save on the cost or and bullying over “in fashion” clothes. However, children need clothes to wear outside of school, and so you really aren’t avoiding that expense anyways. Further, we go all the way back to that first point about bullies. Maybe a student isn’t getting bullied about the way they dress at school, but they sure as hell as getting in on the weekends.
6. Supporters say that uniforms promote safety, like the foolish comment below that says someone in a school uniform is a minor and shouldn’t be flirted with or given alcohol. I don’t know about where the rest of you grew up, but in Canada, clothing can be changed. If by saying that a school uniform tells you that you shouldn’t be flirting with minors, than maybe you shouldn’t have a teaching degree, because that’s the only place where that’s going to take place en masse.
7. Supporters of uniforms say that they force students to express themselves in other ways, but that’s just nonsense. I work with dozens and dozens of students every week who never developed a personality because they were forced by their schools and society to conform. Clothing and appearance are expressions of identity, and stripping that away like students are in prison is wrong.
8. Most adults don’t wear uniforms. With the exception of emergency services (specifically police officers), military personal, and airline staff, very few people actually wear a full uniform in real life. Nurses and doctors wear scrubs, but they are free to choose the fit, make, and colour of those scrubs. Firefighters have special equipment they wear on the job, but are otherwise free to choose what they are wearing underneath. A cashier at 7-Eleven might need to wear an identifying smock or shirt and might even have a dress code, but they can choose how that applies to them. In reality, very few people actually wear a head to toe uniform like those pushed in some schools. Rather, people have dress codes, which are a set of rules for maintaining a consistent and professional appearance. Like a helicopter parent, all a school uniform does is prevent people from developing the proper life skills to be able to decide what is appropriate and what isn’t. If children are never allowed to or required to make decisions in their life, they will never learn how to make them, and that just sets them up for even more failure as adults.

Now, do I believe that schools should have a dress code? Absolutely. But a dress code is not the same thing as a uniform, and there is a wealth of empirical evidence against every single claim people can make in support of uniforms.