Since both Mei and Kouichi had been declared "non-existant", they could basically get away with that, and Izumi is regretting it along with the rest of the class.

Not really. Yes, their class would have to ignore them, but the rest of the people in school can talk to them (the librarian talk to them precisely for that) on school, therefore the director or someone else could expel them. His sister acts as teacher, but outside school she is just his sister, so once she gets back to home, well, she would kick their asses.