I’m back, with two students, to grind (4)
I believe the answer is:
mill
'to grind' is the definition.
(milling is a kind of grinding)
'i'm back with two students' is the wordplay.
'two students' can mean two substitutions for 'student'.
'i'm' becomes 'im' ().
'back' shows that the letters should be reversed in order.
'with' is a charade indicator (letters next to each other).
'student' becomes 'l' (as in L-plates for learner drivers).
'student' becomes 'l' (as in L-plates for learner drivers).
'im' in reverse letter order is 'mi'.
'mi'+'l'+'l'='MILL'
(Other definitions for mill that I've seen before include "J.S. - - , philosopher" , "Fist fight" , "No fun going through this" , "Building for grinding corn into flour" , "works on > race" .)