With this set up, it's entirely possible for someone to have classes a year (or possibly two years) above or a couple years below their class level.  Someone with an "O" in a subject at NEWT level could be a level or two above, someone pretty low say an "A" or lower at the OWL level could take a year or three lower (especially if NEWT level, say, Herbology was required for the job they'd want).