6th and 7th year classes are entirely optional, and are picked based on your OWL scores and what career path you're trying to get into.  It's more like junior and senior years of college than high school so far as how the curriculum is structured.  You're essentially taking all classes related to your "major".<br/><br/>Hogwarts is based on the british secondary school system. (Which, having read some articles on it, I feel makes much more sense than what we have here in America.)