There could be a number reasons for this....Perhaps the upper forms are taught together on a two year rotation. So the trio will learn what the twins learned last year with Moody (Barty Jr), in their next year. Or this may be a study hall like was showed with Snape in the GoF movie. Umbridge combined the class because she had to much to do to teach two NEWT classes. Take a guess one of them is bound to be the reason. LOL