Classes are divided into Beginner (1st & 2nd Years), Intermediate (3rd-5th Years), and Advanced (6th & 7th Years) level classes. Beginners and Intermediates take all six mandatory core classes. First years also attend flying lessons (find these on the Quidditch Pitch); this is not required for second years, though they may opt in if they so choose. Third years may take on optional electives subjects and/or an independent study in a subject of their choice (this must first be cleared with the Headmaster).
At the end of 5th year, students take a CATS (Critical Assessment of Talents and Skills) in each of their subjects. Following this, advanced level students may choose which classes to study, based on what they wish to pursue (and, of course, their grades!). They may choose as many or few subjects as they like, including taught subjects and/or independent study, but at the end of 7th year students take their RATS (Ridiculously Anal Testing of Skills). Two passing RATS are required to graduate, and it is highly recommended to take at least three if you wish to pursue a career or further education.
Study hard, listen well. Magic is a dangerous business.