If you were previously removed from a class or if your previous request to join a class was denied, you cannot join the class again on your own. You'll need to do the following to re-join the class.
First, contact the admin of the class in person to tell them you need to be re-added to the class. Ask the admin of the class to take the following steps:
Step 1: Go to the class page and click on the "members" tab.
Step 2: Click on "Show removed members" at the bottom of the page.
Step 3: Find your username and click on "restore" on the right side of the screen.
You should then be a member of the class again.