Backing some students is a mistake (4-2)
I believe the answer is:
slip-up
'is a mistake' is the definition.
(I've seen this before)
'backing some students' is the wordplay.
'backing' says the letters should be written in reverse.
'some students' becomes 'pupils' ('pupil' can be a synonym of 'student').
'pupils' back-to-front is 'SLIP-UP'.
(Other definitions for slip-up that I've seen before include "Minor lapse" , "Small error" , "Mistake, blunder" , "Minor inadvertent mistake" .)