What makes the Houses have their particular character? the diktats of the Head? that 7th-year students remember what they were taught about the House the last time, they were Sorted into it, 3 years ago, and try to teach the others? I like the idea of putting people into Houses that they have the most to learn from, but then I think that you have to keep the House assignments permanent, or else lose the House characters entirely. (Not that that would necessarily be a bad thing ….)