Expression used for part of school year (4)
I believe the answer is:
term
I believe this clue is a double definition.
'expression' is the first definition.
(both can mean a piece of vocabulary)
'for part of school year' is the second definition.
(I've seen this before)
'used' is the link.
(Other definitions for term that I've seen before include "what being 9 [PREGNANT] leads to?" , "Fixed period of time" , "A period, esp. of schooling" , "Fixed period or special word" , "Academic period" .)