1.There is no student in this school who does not wear a school uniform.
2.There is no student in this school who does not wear the school uniform.
Which article should be used before the word "uniform " in this sentence, a or the?
It really depends. Both can be correct.
If all of the students are wearing the correct uniform for that school, you would use "the school uniform". This means that all of the students are wearing the same version of the uniform.
If you instead say "a school uniform", you may imply that, while all students are in a uniform, they may not all be in the correct uniform for that school. Perhaps a student transferred from a different school and has not yet had time to acquire the uniform for this school.
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Also, schools may offer a choice among a few options in their uniforms, or the dress code might be different for students of different ages, so the school may not have only one single uniform. In that case, "a school uniform" would still work, but "the school uniform" wouldn't. – PMV May 31 '17 at 0:12
