Naughty pupils showing ill manners (3,4)
I believe the answer is:
bad form
'naughty pupils' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'ill manners' is the wordplay.
'ill' becomes 'bad' (similar in meaning).
'manners' becomes 'form' (I am not sure about this - if you are sure you should believe this answer much more).
'bad'+'form'='BAD FORM'
'showing' is the link.
Can you help me to learn more?
(Other definitions for bad form that I've seen before include "Offending against conventions" , "Breach of social conventions" , "Poor behaviour or school class of duffers" , "unruly class" , "Reprehensible conduct" .)