Stupid group of pupils changed side (5)

I believe the answer is:

crass

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'stupid' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Other definitions for crass that I've seen before include "'Gross, vulgar (5)'" , "Showing gross lack of sensitivity" , "Grossly stupid, tactless" , "Insensitively stupid" , "Asinine" .)

I've seen this clue in The Daily Mail.

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