Backsliding students make an error (4,2)
I believe the answer is:
slip up
'make an error' is the definition.
(I've seen this before)
'backsliding students' is the wordplay.
'backsliding' says the letters should be written in reverse.
'students' becomes 'pupils' ('pupil' can be a synonym of 'student').
'pupils' backwards is 'SLIP-UP'.
(Other definitions for slip up that I've seen before include "Maybe literal" , "Make a mistake, on a banana skin ?" , "Small error" , "howler?" , "Make a mistake, being unsteady on feet?" .)