Call time in school (4)
I believe the answer is:
term
I believe this clue is a double definition.
'call' is the first definition.
(to term something is to call it something)
'time in school' is the second definition.
(as in school terms)
(Other definitions for term that I've seen before include "Limit - school time" , "what being 9 [PREGNANT] leads to?" , "Fixed period or special word" , "Expression, or period in which law court holds sessions" , "Division of the school year" .)