To archive a class, start by going into the classroom you want to modify and go into the Manage Class settings. Scrolling down to the bottom will reveal the option to archive the classroom. Doing so however will remove the classroom access from everyone who was permitted.
In order to restore the classroom, you can go into your personal settings menu where you will see an option to Manage Archived Classes
From this menu, you will be able to permanently delete the classroom or restore it granting access back to everyone assigned to it.