I'm against them
Like KDA20 said, it's supposed to stop kids from getting bullied by making it less obvious what kind of an economic background you come from, but you can still tell. And kids definitely still get bullied (just recently the UK, where most schools have uniforms, scored second lowest in Europe for student well being, for example).
The other thing is it's supposedly less expensive for parents to just buy the uniform and then not worry about buying many other clothes, but uniforms tend to be quite expensive and then there's every day clothes on top of that.
Uniforms are a part of the 'hidden curriculum' in schools - ie. socialising kids into capitalist society. They're intended to demonstrate that you 'belong' to the school, and are a homogeneous student body rather than individuals. The uniform distinguishes students from teachers clearly, reinforcing the school's hierarchy. Strict uniform rules are there to be broken. So if a teacher wants to make a point about authority, they can always punish somebody, because it's so common for people's uniform to be 'wrong' in some way. The rules aren't always enforced, they're just there so that teachers can demonstrate their power.
Uniforms are often sexist; with separate uniform rules for girls and boys, which exclude nonbinary individuals.
Another argument is uniforms prepare students for the workplace; but this assumes that all students plan to go on to a specific type of working environment; a white collar, office environment. Which is not the case. Increasingly, also, dress codes for this kind of working environment in the real world are becoming more relaxed anyway. It's outdated to assume that suits, ties, etc. will be a requirement. In addition, schools contradict themselves in saying this as most appear to expect that all students aim to go to university, where there are no dress codes.
Uniforms are often impractical and uncomfortable - for example the school I went to used to force students to wear blazers, and boys to wear shirts with their top buttons fastened, in 30 degree heat. Which is clearly unhealthy and unhygienic.
It's a myth also to assume that without uniforms clothing would be a huge focus and source of worry for students. This is the case for students at schools with uniforms on non uniform days because it's a special occasion, so they feel the need to make an effort and dress up. If non uniform were the norm, what someone was wearing would not be scrutinized like that