What should I do about my Ed-Visa extension if I finish my courses early?

You need to ask your school to write a letter saying when the end of your studies are. Whatever the date on this letter is, is when you need to cancel your extension by that date. If they will write you a letter that says Dec 17, or Dec 7, then you can use that.

Regardless of what the letter says, you need to take it and go to immigration about a week before that date to cancel your ED extension. They should cancel it for you then stamp you in for the extra few days until the date on the letter. Then you have until that date to leave Thailand.

You signed a document saying you would inform immigration if your status changed and finishing your studies is a change of status. If you don't cancel your extension, even if you leave after the date runs out, you've failed the agreement you signed and you'll not be able to get another long term extension in Thailand until you get it resolved and you'll also be fined 20,000 baht when you try to do so.