It really does vary from school to school, of course, but here in California public schools each student is required to get a certain amount of credits in each subject, necessitating a certain amount of semesters/years for each type of course. Generally, the result of this system is that early on in one's high school career (and when one enters middle school) most of your classes are already laid out for you, except that you occasionally may test into or choose a more advanced course for the required subject. For example, freshman year you will automatically be placed in Freshman English, and you have to take biology for your science course, but you are permitted to choose between Honors Biology and the regular biology class. Later on as you become an upperclassman you have more freedom to organize your class choices provided you still gain the amount of credits in each subject required to graduate. Some people choose to complete the minimum number of required subjects (English, languages, visual arts, etc.) as early as possible and then fill their senior year with courses that are considered "easy", such as vocational courses like auto or home ec or shop, though this isn't necessarily advisable if you want to attend a good university. ;P