If students are already in one of your classes, you can add them to another class with a few clicks! Students won't need to enter a join code or do anything on their end.
Go to your "Classes" tab.
Click the dropdown menu carat next to the class name of the class your students are currently in to bring up the student list
Select the students you'd like to move by clicking on the bubbles to the left of their names
Choose "add to classes" from the menu at the bottom of the screen
Select which class to move them into
Click Add :
Voilà! Your students will now be in your preferred class(es).
Note: If you do not want the students you moved to remain in the original class, simply delete them!