Archiving a class will prevent it from showing up on your Classes page or in your Reports, but without deleting the class and it's data. This can be useful for older classes where you want to save them, but won't be using the them going forward. If you ever needed to access the class, you always have the option of unarchiving it.
To archive a class, click on the menu dropdown arrow and select “Archive”.
To unarchive a class and restore its visibility, click on the “Show archived classes” link at the bottom of your Classes page. Select the menu button from the class and choose "Unarchive".