Upcoming student’s mistake (4-2)
I believe the answer is:
slip-up
'mistake' is the definition.
(I know that slip-up is a type of mistake)
'upcoming student's' is the wordplay.
'upcoming' is a reversal indicator.
'student's' becomes 'pupil's' (pupil can mean a student at a school).
'pupils' back-to-front is 'SLIP-UP'.
(Other definitions for slip-up that I've seen before include "Mistake, blunder" , "Minor inadvertent mistake" , "Small error" , "Minor lapse" .)