Extra note about ordinary day in schools (4)
I believe the answer is:
pods
'schools' is the definition.
(both can mean groups of marine animals)
'extra note about ordinary day' is the wordplay.
'extra note' becomes 'PS' (a PS or postscript in a letter).
'about' means one lot of letters goes inside another.
'ordinary' becomes 'O' (abbreviation e.g. O-Levels).
'day' becomes 'd' (abbreviation for day).
'o'+'d'='od'
'ps' going around 'od' is 'PODS'.
'in' is the link.
(Other definitions for pods that I've seen before include "Pea cases" , "some seals" , "schools of whales?" , "Groups of dolphins" , "Seed containers" .)