By getting certain school exercises right (3)
I believe the answer is:
per
'by' is the definition.
(I've seen this in another clue)
'certain school exercises right' is the wordplay.
I cannot quite understand how this works, but
'exercises' could be 'pe' (Physical Education) and 'pe' is present in the answer.
'right' could be 'r' (abbreviation) and 'r' is located in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'getting' acts as a link.
Can you help me to learn more?
(Other definitions for per that I've seen before include "Preposition meaning for each" , "For each or every" , "According to" , "Preposition indicating by means of" .)