You don't have to, but many subjects are covered through a sequence of courses. Let's use math as an example. Some students will start with Refresher Math, move on to Algebra 1, then on to Geometry, then on to Algebra 2, etc. Students move through courses at their own speed, and then go on to the next level when they're ready. All Keystone courses are designed to balance the introduction of new information with the repetition of learned information.