Hi Sarah
If I understand your requirement correctly, you can use two payment plugins come with Events Booking by default:
1. Paypal (so that registrants can pay using Paypal when they register for the event)
2. Offline payment (so that registrants can pay at a later time, in your case, pay when they come and attend the class)
So just go to Events Booking, publish Paypal and Offline payment plugins, unpublish the others and It will work well
Tuan