Teaching assistant in school going back to mark (6)
I believe the answer is:
notate
'to mark' is the definition.
(to make a note)
'teaching assistant in school going back' is the wordplay.
'teaching assistant' becomes 'TA' (employee in UK school).
'in' means one lot of letters goes inside another.
'school' becomes 'eton' (famous UK public school).
'going back' shows that the letters should be reversed in order.
'ta' placed into 'eton' is 'etaton'.
'etaton' back-to-front is 'NOTATE'.
(Other definitions for notate that I've seen before include "Compose" , "Record, say" , "Write in system of signs or symbols" , "Express in symbols" , "Write symbolically" .)