Institutions that train large groups of swimmers (7)

I believe the answer is:

schools

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'institutions' is the definition.
(school is a kind of institution)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for schools that I've seen before include "Institutions of learning" , "Colleges" , "Places of instruction" , "Educates" , "Institutes of learning" .)

I've seen this clue in The Globe and Mail.

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