No, they should not be mandatory
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School Uniforms are impractical
They are not fit for purpose. School uniforms are often designed to fit all body types, which means that they may not be flattering for many people’s body forms. The material of school uniforms is another factor that could add to the discomfort; uniforms can be made using either cheap canvas or polycottons.
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School uniforms are not designed well for the pursuits a child undertakes at school.
Depending on the age of the child, a child’s school day will likely involve some form of creative arts and crafts, an hour of active play in the playground, then a lengthy portion of the day sat on a chair. The perfect clothing items for these rugged, mucky, sticky, sweaty activities would be loose-fitting, inexpensive, and comfortable. Yet most school uniforms are the exact opposite. They are essentially formal clothes, often requiring students to wear ties and slacks. They are in no way fit for purpose. When they get ruined, which they inevitably will, parents have to buy expensive new shirts, shoes, blazers, ties, and shoes, only to have them get ripped, stained and torn to shreds all over again. In an effort to keep costs down, uniforms are often made from cheap materials, making them even more unfit for learning. Polyester shirts and cheap cotton blazers often make children hot in the summer, while offering no insulation in the winter. There is no way of solving this issue. If schools made uniforms out of better-quality materials, they would be expensive and put an unnecessary financial burden on parents. But keeping them cheap and affordable, means they are impractical. The only way out of the trap is to allow children to wear their own clothes to school. 
When children are allowed to choose their own clothes to wear to school, they wear even more impractical clothing. In schools without a uniform, children and teens always choose fashion over practicality. The clothes they wear are usually selected because they look cool, not because they are conducive to learning or fit for the rigours of the school day. Therefore, when compared to the alternative, school uniforms are the practical choice. Although not ideal, they are suited to both classroom and playground environments and should, therefore, be a mandatory part of school life. 
[P1] School uniforms are not fit for school life. [P2] Therefore, they should not be mandatory.
Rejecting the premises
[P1] They are better suited to school life than most of the clothes children wear when no policy is implemented. [P2] Therefore, in the interests of practicality, they should be a mandatory requirement.