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You can access the late submit feature from the course page assignment itself, (on the left side bar under both assignments, and students, (push the three horizontal bars in the upper left side to expand it if it's hiding))--
--or from the assignment itself, again on the left side bar (under both assignments, and students):
While we expect this machinery to behave as set should a student report a problem of not having access when they should after being granted a late submit, please toggle their time forward just by a minute, and then back to the original (extended) time. This resolves the problem the vast majority of time. Any additional help may be received at questhelp@austin.utexas.edu
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