Taking students in retirement is a mistake (4-2)
I believe the answer is:
slip-up
'is a mistake' is the definition.
(I've seen this before)
'taking students in retirement' is the wordplay.
'taking students' becomes 'pupils' ('pupil' can be a synonym of 'student'. I am not sure about the 'taking' bit.).
'in retirement' says the letters should be written in reverse (retire can mean to go back or backwards).
'pupils' back-to-front is 'SLIP-UP'.
Can you help me to learn more?
(Other definitions for slip-up that I've seen before include "Minor inadvertent mistake" , "Minor lapse" , "Mistake, blunder" , "Small error" .)