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?