Lasting impression of the extremes of the school year (4)
I believe the answer is:
scar
'lasting impression' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'of the extremes of the school year' is the wordplay.
'of' becomes 'sc' (I can't justify this - if you can you should give a lot more credence to this answer).
'the extremes of' says to hollow out the word (remove centre letters).
'the school year' becomes 'academicyear' ('academic year' can be a synonym of 'school year').
'academicyear' with its middle removed is 'ar'.
'sc'+'ar'='SCAR'
Can you help me to learn more?
(Other definitions for scar that I've seen before include "Stigma" , "It shows where wound was" , "nine [AL CAPONE] had to face" , "Mark left by the healing of injured tissue" , "Steep cliff; skin mark" .)