LearnDash is the most popular LMS (Learning Management System) plugin for WordPress: Multi-level courses; Sell courses; Certificates and Points; Advanced Survey; Expire Access; Task Management; Dynamic forums; Lesson Timers; Flexible Prerequisites; User Profiles;

LearnDash comes with more than a few levels that make it easy to create courses, lessons, topics, quizzes and categories. It has progressive playback functionality so that you can program the course content for the duration of the entire course.

You can assign users and then approve or comment on them. It also comes with a quiz and testing manager tools with multiple questions. You can easily identify the prerequisites so that each student can move on to the next class.

There are several payment gateways available to accept payments from PayPal, Stripe, 2Checkout, etc. It also fits beautifully to bbPress.