Stressful experience getting low grades in exam (6)
I believe the answer is:
ordeal
'stressful experience' is the definition.
(I know this)
'low grades in exam' is the wordplay.
'low grades' becomes 'de' (D and E are low academic grades).
'in' indicates putting letters inside.
'exam' becomes 'oral' (spoken academic test).
'de' put into 'oral' is 'ORDEAL'.
'getting' is the link.
(Other definitions for ordeal that I've seen before include "A tough time or hardship" , "A difficult and harrowing experience" , "Difficult experience, a tough time" , "A tough and testing time" , "Difficult or arduous experience" .)