Part of school day — full stop! (6)
I believe the answer is:
period
I believe this clue is a double definition.
'part of school day' is the first definition.
(division in a school timetable)
'full stop' is the second definition.
(US name for a full stop)
(Other definitions for period that I've seen before include "Indefinite length of time" , "Duration or interval" , "Stretch of time" , "Amount of time - US full stop" , "End of story" .)