If you want Xavril to have taken Astronomy, then you can go back and change it now, but once you take the class you are stuck with it.  Bear in mind that each class will only be used every so often (per year), so 6th year Astronomy probably won't be seen again for a while, same for fourth year Wandlore.  Some classes will be seen more than others, as some are just easier for me to write lessons for.<br/><br/>In addition, I also try to schedule classes based on when I think it would be more interesting to see certain professors, hand out certain assignments, or reveal something about a certain student.  I change my "schedule" around constantly.  Expect each year to have no more than one conventional class a day.