LouiseCollins The uniforms force very young kids to dress in a gendered way. In general girls choose skirts for the first few years in primary school, because that is the prevailing fashion for girls, and you have to choose between 2 or 3 possible outfits. If you look at kindergarten kids in Europe and much of the USA, there is far less conformity- boys and girls in t-shirts and soft trousers of all colours and cuts.
The reason that uniforms force people to think about clothes is that the school is always telling kids how to dress, making rules and judgements. When schools simply stop mentioning clothes altogether, they become less of a way to rebel or to assert yourself, and can just be a natural part of a growing person's identity.
My children and their friends, in a big city in the South of England, wear far more makeup, far less practical clothing (like warm coats and shoes) and have much worse judgement and less confidence about their appearance, than my niece and nephew and their friends in NYC, or all of our friends in Germany and the Netherlands. The foreign schools do not ever comment on students' appearance, and as a result, it is less of an issue.
I know that the theory of uniforms is that everyone just wears the same thing so no one can tell your income, and thinking about appearances doesn't interfere with learning. The reality I have observed (3 kids of 22, 19, and 16) is that uniform policy backfires and creates a culture which is overly obsessed with appearance and lacking in common sense.