Reviewing student's mistake (4,2)
I believe the answer is:
slip up
'mistake' is the definition.
(slipping up is a kind of mistaking)
'reviewing student's' is the wordplay.
'reviewing' says the letters should be written in reverse (I've seen 'reviewed' mean this).
'student's' becomes 'pupil's' ('pupil' can be a synonym of 'student').
'pupils' written backwards gives 'SLIP-UP'.
(Other definitions for slip up that I've seen before include "Pupils (anag.)" , "Make a mistake, being unsteady on feet?" , "one fielder close to the wicket?" , "Mistake, blunder" , "get it wrong" .)