Tell school to accept fool (6)
I believe the answer is:
snitch
'tell' is the definition.
(I've seen this in another clue)
'school to accept fool' is the wordplay.
'school' becomes 'sch' (abbreviation).
'to accept' means one lot of letters goes inside another.
'fool' becomes 'nit' (both can mean a stupid person).
'sch' going around 'nit' is 'SNITCH'.
(Other definitions for snitch that I've seen before include "Whistleblower (informal)" , "who tells on his classsmate?" , "Inform on someone in slangy way" , "I'm telling!" , "Inform on, tell tales" .)