noblegirraff (a great name, I love giraffes) I actually believe in school uniform. I think a simple, basic uniform is a good idea, it can set a school identity and make life easier for pupils and parents. If it is not overpriced or over the top (top hats and tails etc would be over the top!)
However, if a large amount of teacher time is taken up with enforcing the uniform I feel this is bad and helpful. I do not believe it is every teachers' job to speak to every pupil about their uniform.
Since most schools don't seem to wear a sign about the pupils, I am guessing it is not a universal issue.
A sign can be shaming. It may not intend to be, but it can be.
It appears you only need a sign if you think it is the job of every teacher to be constantly vigilant about every pupils 'attire'. Even if this means only noticing an infringement and then reading a lanyard that is a massive waste of time and energy, surely?