Turning heads in fifth form, including teacher's pet (4)
I believe the answer is:
tiff
'pet' is the definition.
(I've seen this in another clue)
'turning heads in fifth form including teacher's' is the wordplay.
'turning' is a reversal indicator.
'heads in' says to take the initial letters (I've seen 'head in' mean this).
The initial letters of 'fifth form including teachers' is 'ffit'.
'ffit' written backwards gives 'TIFF'.
(Other definitions for tiff that I've seen before include "Petty argument" , "Hard to lose opening argument" , "causing quarrel" , "Lovers' row" , "Quarrel (colloq.)" .)