To archive a class, go to your "Manage Classes" page. Click on the "Edit Class" button for the class you wish to archive. A small window should appear with the class's information. Click the "Archive Class" button at the bottom of the window. Your class and all of its assignments will then be removed from "Active Classes" and saved in the "My Archived Classes" page.
If you would like to view any of your archived classes later on, you can do so by clicking on the "Archived Classes" button on the "Manage Classes" page.
You can also restore or undo an archived class by clicking on the "Undo Archive" button in the "My Archived Classes" page.