There are some great ways the system can help you do this.
Track who is not showing up to classes
Send a pre-class reminder email
How about sending a quick prompt before the class starts, reminding customers to unregister if they can no longer attend?
Send an automated email to customers who no show
For instance, you could send customers an automated email when they no show. See the below guide for where to head to do this.
Note. You'll need to set up a post class trigger sent to no shows only.
Add a fee to the customer's membership
Maybe you want to fine customers for now showing over a certain number of times.
For class packs you could subtract some of their usage: