If the class you wanted to take is full, you can join a waitlist. Simply tap Join the Wailist and your teacher will be notified.
You can confirm you are waitlisted by going to the bookings page of your account:
You can remove yourself from the waitlist by tapping Cancel to the right.
When a spot opens up you will be notified over email that the spot is available or that you have been added automatically depending on your teacher's business settings.