Standards of performance by students (6)
I believe the answer is:
grades
'standards of performance by students' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for grades that I've seen before include "Numbers or letters indicating quality" , "Standards of performance awarded to students" , "Ranks in order" , "Ranks or classes" , "Levels of proficiency or value" .)