Leave suddenly, affected by low grades? (6)
I believe the answer is:
decamp
'leave suddenly' is the definition.
(I've seen this before)
'affected by low grades?' is the wordplay.
'affected' becomes 'camp' (camp can mean exaggerated or affected).
'by' says to put letters next to each other.
'low grades?' becomes 'de' (D and E are low academic grades).
'camp' after 'de' is 'DECAMP'.
(Other definitions for decamp that I've seen before include "Make off secretly" , "Abscond" , "Split" , "Make off - from tent?" , "Bolt" .)