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You may need to reload/refresh the page for the student to appear in your list of students. If a student does not appear, it could also be because the student used the wrong code (make sure there are no empty spaces before or after the code).

If you use Google Classroom or Clever and have already invited your students, you can always "resync" by clicking on 'Invite students'. Useful if a student has subsequently joined one of your classes.