Rotting group of fish -- there are lessons to be learnt here (4,6)

I believe the answer is:

high school

'there are lessons to be learnt here' is the definition.
(where pupils learn things)

'rotting group of fish' is the wordplay.
'rotting' becomes 'high'.
'group of fish' becomes 'school' (I've seen this in another clue).
'high'+'school'='HIGH SCHOOL'

I've seen this clue in The Telegraph.

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