Equity?
The 'equity' argument goes like this: If children wear uniforms, they do not notice differences between children from rich and from poor families. This 'equity' argument is often put forward by State Schools. The reason for this may be that it is a purely socialist argument and it may be rejected for this reason alone. In a democratic country, school should not indoctrinate children with a specific political ideology, especially not a government-funded school. Interestingly, private schools typically are even more fanatical about uniforms, but they are less inclined to use the 'equity' argument.
Anyway, even as a socialist argument, it does not make much sense. School uniforms may make all students look alike. But why do the teachers not wear the same uniforms? Clearly, school does not like any confusion as to who is the teacher and who is the student. The master-slave relationship that is so obviously present at school is deliberately magnified by uniforms that emphasize this difference. The teacher is allowed to dress casually, while the student has to wear silly clothes intended to make the student look stupid.
Furthermore, there are often different uniforms for those in higher grades than for those in lower grades, just like in the military a superior officer wears a less silly hat. This creates class differences. Some will argue that this merely reflects existing differences. But the point is that if this were accurate, it constituted an argument against uniformity. Moreover, school itself creates class differences. Class is a trademark, if not an invention of school. Children are grouped together in classes according to age and often according to gender and to perceived academic performance. Because parents want their children to mix with children of their 'own class', they carefully select the neighborhood where they are going to live. Houses close to private schools are often substantially more expensive than similar houses close to state schools. On the street, children are identified by their uniform. 'Oh, you come from that poor school, you dummy!' is an example of what children say to each other when they look at each other's uniform. And even in the classroom, uniforms only accentuate differences in length, hair color and other physical characteristics. Children consequently judge each other by their physical appearances. One can argue whether it were better if children judged each other by their clothes instead.