How does Sumac enforce registrants to pay in full before a course starts?
When defining sessions in the Field Values area, you are able to specify the cost of any course you offer, as well as a minimum deposit required to register. You could indicate that the minimum deposit required is the full amount for the course. This way, registrants would be required to pay in full at the time of registration, ensuring that all registered students are fully paid up at the time the course starts. Otherwise, Sumac cannot do anything to prevent people from attending a class they have not paid for.
If you do not require the full amount of the session to be paid at the time of registration, there is a report to help you track deposits. When using Sumac in the Course Registration - Registrations View area, there is a report called Class List With Financials that shows all the students registered for a class and how much they owe.