In some countries, Harry, Muggles have barristers who argue cases in courts of law. Frequently, a barrister will say something or do something, knowing full well that the judge will order it stricken from the record. It doesn't matter; even if the jury are told to disregard what just happened, they can't. Not completely.

Even if they force you to return to Slytherin tomorrow, no one in the school will be able to ignore what you've just done. You'll have made it part of their record.

Meanwhile, get some sleep. You'll need it to stay strong tomorrow.