Trouble over, for example, a pupil? Could be against the rules (7)
I believe the answer is:
illegal
'be against the rules' is the definition.
(I've seen this before)
'trouble over for example a pupil?' is the wordplay.
'trouble' becomes 'ill' (synonyms).
'over' means one lot of letters go next to another.
'for example' becomes 'e.g.' (synonyms).
'pupil?' becomes 'l'.
'ill'+'eg'+'a'+'l'='ILLEGAL'
'could' is the link.
(Other definitions for illegal that I've seen before include "Ignoring certain acts" , "Contrary to law" , "Forbidden" , "Criminal, illicit" , "murder sure is!" .)