You could mention this in the feedback when grading the user's homework. This way they will remember it for their next homework (:

To keep it realistic, it is not possible to just change the house, since in the books the students aren't able to do that either. If you want to be sorted in a specific house, it is rather easy to answer the questions of the sorting hat in a way that will make him place you in your house of choice.

Most of the exams can be passed by just googling the questions, so you don't necessarily have to attend the lessons or do the homework to pass your exams. Also, to keep it realistic, exams are necessary to move to the next year.