Thursday Morning Riddle

In many long sentences, I’m at the end;
I’m a third of a hockey game; her monthly friend;
An emphatic pronouncement you cannot amend;
And the time in each class that a student must spend.

Who am I?

UPDATE: Riddle solved by Asher. See comments for answer.

2 Responses to “Thursday Morning Riddle”

  1. Asher Says:

    PERIOD

  2. Bill Says:

    Asher is correct… PERIOD.

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