1. Login to the district admin account.
2. Go to Manage District in the left toolbar.
3. Go to the Classes tab at the top.
4. Select the classes using the check boxes.
5. Use the option on the right top screen which says actions to choose Archive selected classes as below.
6. Now, you will get a pop-screen asking you to confirm the action by typing "ARCHIVE" in the box.
7. Click on "Yes, Archive>" and the selected classes will be archived.