Place of learning, possibly of thought (6)

I believe the answer is:

school

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'place of learning' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

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(Other definitions for school that I've seen before include "Large group of fish" , "company that's all at sea" , "For the education of fish" , "A number of fish" , "Group of sea creatures" .)

I've seen this clue in the Evening Standard.

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