Once a quiz has been submitted in Classic quizzes there is no way to open it back up for students. The only option is to give them another attempt. I know some people when they do this ask their students to just do the questions they didn't have time to complete on that 2nd attempt. As the teacher you will be able to see every attempt in Speedgrader so you can manually combine the results from both to get a correct score. It's not ideal, but it does work.
Using Moderate works to provide additional time before students begin a quiz and you can also provide more time to a quiz in progress, but it can't extend the time of an already submitted quiz.
Rick
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