Not a teacher!
I believe the expression "calling time" originates from a time when pubs in Britain had to close at 11PM. Then the landlord (publican) would shout (call), "Time, gentlemen!" a few minutes before 11PM, thus allowing every punter to order a last pint at the bar before closing time. At 11PM a bell would ring and the roller shutters would go down on the bar.
Here is a dictionary definition:
call - Definition and pronunciation | Oxford Advanced Learners Dictionary at OxfordLearnersDictionaries.com
call time (on something) (British English) to say or decide that it is time for something to finish
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