A violation of students freedom of expression
School uniforms restrict students' basic right to freedom of expression.
Many religions have strict dress codes. The rigidity of mandatory school uniform policies means many religious students are forced to choose between following school rules and adhering to their religious principles.
Uniforms can be at odds with students’ religious dress codes. This can cause students to be forced to choose between adhering to the school’s uniform code, or their religious dress code. Schools are quick to proclaim that in cases of religious principles and expression; they will make an exception to the dress code. However, this often causes a host of additional problems. Students that are exempt from the school uniform regulations due to religious principles are vulnerable to prejudice and discrimination. It can lead to the isolation of religious groups, bullying and an increase in cultural intolerance. 
Most countries and states which allow for school uniform policies to be implemented have sufficient safeguards to protect religious freedoms. A school’s ban on religious clothing or religious attire would infringe a student’s right to religious practice, and therefore cannot be enforced. School uniform policies have wiggle room to allow for religious freedom of expression. Schools that have uniform policies prohibiting hats, for example, allow students who wear hijabs or yarmulke’s in the interest of protecting religious freedoms. Therefore, a mandatory uniform policy is clearly compatible with religious freedom of expression.
[P1] Uniforms violate religious freedoms. [P2] Therefore, they should not be mandatory.
[Rejecting P1] Mandatory uniform policies are always flexible enough to allow for religious freedoms.