Steph30,
Hello.
In a different sentence, I would be inclined to say:
"British soldiers wear a uniform".
So:
"English pupils wear a uniform". This suggests that all the pupils in School X wear the same uniform, the School X uniform.
However, although I would not find a serious problem with someone who said:
"English pupils wear uniforms", this sentence suggests that each pupil has more than one uniform, and that each uniform is different. It also suggests that pupils from different schools wear different uniforms.
In the second example, there are many children, and there are many different uniforms. For the above reason, I would say:
"They are wearing uniforms", (and they are all different uniforms!)
Otherwise, you would have to say:
"Every one of them is wearing a uniform", (and every one of the uniforms is different).
I hope that I have helped you.
Les
