Swimming school (5)

I believe the answer is:

shoal

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This could be a double definition.

'swimming' is the first definition.
Both the answer and definition are adjectives. Perhaps there's an association between them I don't understand?

'school' is the second definition.
(synonyms)

Can you help me to learn more?

(Other definitions for shoal that I've seen before include "Lots of fish" , "School of fish" , "Collection of fish" , "Mob" , "Fish grouped" .)

I've seen this clue in the Evening Standard.

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