Differences in clothing can serve to highlight differences in

Conclusion: By requiring students to wear uniforms, schools make it so that the way students are dressed when they go to school no longer highlights differences in financial means.

The conclusion is about the way students are dressed when they go to school.

C says that students going to schools that require the wearing of wear uniforms only when they are either in school or going to and from school. So, while C may seem to indicate that students wear something other than uniforms, C confirms that the way students are dressed WHEN THEY GO TO SCHOOL is in uniforms.

Therefore C is in line with what the argument has stated and changes nothing, and therefore C does not weaken the conclusion.

C is a trap answer designed to seem to weaken the argument. It sounds as if it makes a difference, but really it does not.