When you first start the game, you'll only be able to choose Grade C courses, which are the easiest to complete. However, if you manage to beat the Grade C courses, you will unlock Grade B. Should you finish all of the Grade B courses, you'll be able to play the Grade A courses, which are the hardest in the game.

Effect - Code

Grade A Courses - Complete all Grade B Courses
Grade B Courses - Complete all Grade C Courses