What may enlarge as it gets dimmer students (6)

I believe the answer is:

pupils

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'students' is the definition.
('pupil' can be a synonym of 'student')

I don't understand how the rest of the clue works.

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(Other definitions for pupils that I've seen before include "Apertures in the eyes" , "Learners" , "Eye openings" , "The charges" , "Scholastics in the eye" .)

I've seen this clue in The Times and The Sunday Times.

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