Since you're leaving tonight you don't have any/many options. Your school can theoretically cancel your work permit for you, I think they have 15 days. As far as immigration goes there's nothing you can really do tonight. If I read your post correctly the school changed the August 31st date to today's date? I'm not sure how immigration will treat this 1 week overstay, or if it even qualifies. 4 years is a long time and I believe it'll only affect you if you try and get another non Imm class visa.
Maybe
@Tod ********
can weigh in on this.