Not the best pupils, presumably (6,5)
I believe the answer is:
second class
'not the best' is the definition.
'pupils presumably' is the wordplay.
I cannot really understand how this works, but
'pupils' could be 'class' (I've seen this before) and 'class' is found in the answer.
The remaining letters 'second' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for second class that I've seen before include "Of inferior status" , "Lower postal category" , "Slower postal rate" , "B-rated" , "Next to best" .)