I noticed the same phenomenon when I went to a public elementary school that required uniforms.

Even though the school was in one of the poorest areas of the city, kids still found plenty of ways to signal their status to others. High-status kids had more jewelry, fashionable haircuts, and were exempt from many of the uniform rules (such as having to tuck in their shirts) because they made friends with the administrators. Girls tended to signal more with their clothes because we had the options of blouses, skorts, and skirts in addition to the polo shirts and shorts the boys wore.

It took me a few years to understand the subtleties of what was happening, but by my final year in the school, I was playing the status game as well. If one of the goals of having uniforms at that school was to emphasize student equality, I never got the sense that it was accomplished.

This was the case for me in my uniforms required school. The obvious and conspicuous item we could control was our tie, but thinking back on it now, kids signalled identity and status through shoes, belts, and other accessories (though I was effectively blind to such things at the time).

Seniors were also allowed to wear khaki pants, a conscious allowance on the administrators' part designed to reinforce the different classes.

Even these guys have different clothes and such to signal status, as is apparent if one attends events like this.

Once the uniform gets strict enough, there is a limit to how much money can be spent on signalling, though not effort, I think. Things taking effort tend to be subject to diminishing returns, the part of diminishing returns that isn't usually focused on is that one can usually do better by putting more into it.