Edition by school ma'am left out (6)
I believe the answer is:
missed
'left out' is the definition.
('miss' can be a synonym of 'leave out')
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for missed that I've seen before include "Didn't catch" , "Failed to perceive" , "Didn't attend" , "Felt the loss" , "failed to see" .)