Notify when attending class (6)
I believe the answer is:
inform
'notify' is the definition.
(I've seen this before)
'attending class' is the wordplay.
'attending' becomes 'in' (I've seen this in other clues).
'class' becomes 'form' (form class is a kind of class).
'in'+'form'='INFORM'
'when' acts as a link.
(Other definitions for inform that I've seen before include "Spill the beans" , "Give information to (someone)" , "Impart knowledge" , "Tell (someone)" , "Let people know" .)