Error made by returning students (4,2)
I believe the answer is:
slip up
'error' is the definition.
(I've seen this before)
'returning students' is the wordplay.
'returning' shows that the letters should be reversed in order.
'students' becomes 'pupils' ('pupil' can be a synonym of 'student').
'pupils' in reverse letter order is 'SLIP-UP'.
'made by' is the link.
(Other definitions for slip up that I've seen before include "Maybe literal" , "Mistake, blunder" , "err" , "one fielder close to the wicket?" , "Make a mistake or lose footing aloft" .)