www.theguardian.com/education/2017/jul/17/school-makes-pupils-wear-signs-if-uniform-doesnt-meet-standards
The idea being that by wearing a sign, it makes teachers aware that the pupil is aware of the issue and is going to get it sorted.
But I think that wearing a sign just also highlights issues and makes the pupils a potential target.
There are loads of reasons why a uniform might not be up to scratch in the morning. Not all of them are under the pupils' control.