I'm preparing to build a web site that has a teaching application. The
application will have the participants take a series of courses, in
sequence, until completion.

Basic Features List:

Account based - user will pay for the services and create an account.

User will log into their account where they can view the list of "classes"
they must complete.

Each class will be presented as web pages of viewable content.

Each class must be taken in sequence (prevent user from skipping ahead).

After the user has completed the course they will be given an online test.

If the user does not successfully complete a class in a certain amount of
time(days) the user will receive an automatic email reminder.

Thats the basic concept. Does anyone know of some applications, extensions
or tutorials that can help me get started?

Thanks, T