@cgripley
Students may turn in submissions, including quizzes, after the due date and after the instructor has put in a grade unless the instructor adds an "available until" date to stop them. Adding an available until date will keep students from submitting the assignment or quiz.
While your use case is common, there are some faculty like to put in zeros for all of the grades at the beginning of the term and then the student works towards earning their grade. Others put in zeros to gently nudge the students that they need to complete the quiz (for some reason, sending them a notice that the grade is missing is not as effective as putting in a zero). Others want students to continue to turn it in over and over until they get it right. None of those would be possible if students were unable to submit after a grade was entered. Canvas lets faculty have it both ways, but that means that those who don't want to allow students to resubmit need to fill in the available from date.
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