Expression used for part of school year (4)
I believe the answer is:
term
I believe this 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' acts as a 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" .)