If a class is full, how can the student get onto the waitlist online?

Go to Tools > Online Web Registration > Settings (on left) > Class Search Settings and set Allow Waitlist When Class Full to Yes

Check to be sure there is a number of students allowed to be wait listed for each class. This is located on the Class Summary tab > Enrollment > Max Wait. This will provide an online Waitlist link on your Web Registration Form, Parent Portal, and on Class Listings Tables when the class if full.