Turn to welcome couple of graduates in old school (4,5)

I believe the answer is:

alma mater

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

I don't understand the rest of the clue.

(Other definitions for alma mater that I've seen before include "Where one learnt" , "Where one learned" , "College one attended" , "Former college" , "College (person) attended" .)

I've seen this clue in The Hindu.

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