A violation of students freedom of expression
School uniforms restrict students' basic right to freedom of expression.
School uniforms are an expensive, unnecessary purchase which puts an added financial burden on low-income families.
School uniforms are an expensive purchase for many parents. They require parents to buy clothes, often at deliberately inflated prices, that will not be worn at weekends or outside school. This means that parents will also have to purchase additional clothes for the evening and weekends. When you factor in that many families have multiple school-age children, school uniforms constitute a significant expense per annum. If school uniforms weren’t mandatory, parents would not need to fork out. They could send their child to school in the same clothes they wear on the weekends. This would allow them to spend that money on more important aspects of their child's education, like additional tutoring or school trips.
This isn’t an argument against school uniforms, it is an argument against making parents pay for them. Surveys indicate that when parents are asked whether or not they would support school uniforms if the school had to provide them for free, they typically support uniforms. This indicates that it is not the mandatory uniform policies that are a problem, but the financing of the policy.  There is nothing wrong with mandatory school uniform policies. However, to avoid placing an additional financial burden on families, they should be subsidised or paid for by the school rather than the parents.
[P1] School uniforms are expensive. [P2] Mandatory school uniform policies put financial pressure on low-income families. [P3] Therefore, they should not be required.
[Rejecting P2] They do not have to put pressure on families. [Rejecting P3] This argument is not against school uniform policies, merely how they are funded.