Most Western democracies were founded on values of freedom of expression and freedom of speech for all citizens. These are inalienable rights granted from birth.
We are not allowed to force another citizen into conformity by dictating what they can or cannot wear, how they must wear their hair, or what jewellery they may or may not want to wear. Freedom of expression is enshrined in many countries constitutions. Clothes are an important vehicle through which we express ourselves, from wearing a rainbow-coloured T-shirt during gay pride, to wearing a pink T-shirt in support of breast cancer. Mandatory school uniforms are a clear violation of a citizen's freedom of expression.
Clothing is not a student's sole creative and expressive outlet. By taking clothing out of the equation, it actually encourages them to express themselves in other ways such as art, music, literature, or sports. It helps students learn that they are more than just their appearance and the clothes or brands they wear and reduces their reliance on their appearance as their sole form of personal expression. Even in schools with mandatory school uniforms, students are still able to add their own accessories and express themselves through their appearance in that way. It does not limit the child's expressive freedoms in any way. Also, in the workplace, an employee is expected to wear clothes which comply with the company's dress code. If this is permissible under freedom of expression rights, why would school uniforms be a violation of these rights?
[P1] Freedom of expression is an inalienable right granted at birth. [P2] Mandatory school uniforms restrict freedom of expression. [P3] School uniforms should not be mandatory.
[Rejecting P2] School uniforms do not restrict freedom of expression.