There are exactly two reasons why schools mandate uniforms: greed and/or authoritarian leanings.
I can think of three advantages of uniforms:-
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They reduce competition for expensive clothes and build solidarity.
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They are usually cheap and durable, suited for children who will get their clothes dirty (and occassionally fight).
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It is.easier for people to identify if a students gets lost (particularly when outside school).
Of course, for this to work the uniforms should be simple, affordable and comfortable. And they should be uniform - no gendered outfits or special clothes for some students.