What are my options for extending my ED visa in Thailand before it expires?

So to re-cap, what you'd do is

take paperwork from your school stating you stop studying on the 11th to the immigration office, they will cancel your current extension and stamp you in until the 11th.

You can go early, like this week if you want to and they'll still stamp you in until the date on the paper from the school. Then you have until that date to leave the country and either go to a thai consulate and get a single entry tourist visa, OR just border bounce and get a 30 day visa exempt entry stamp.

technically because you're on an extension of stay you need to go to the immigration office and get the stamps transferred to the new passport AND you should also cancel your ED visa extensions (you're not on renewals, you're on extensions of stays).

Failing to cancel it could cause you problems when you get a new visa and go for an extension. In fact some borders won't even let you out of the country to border bounce until you've canceled the existing extension. Just letting it expire does NOT cancel if officially.

I'd say cancel your extension and then go to the border bounce out and back in for a 30 day visa exempt entry (IF you haven't gotten TWO 30 day visa exempt entries by land this year ;) )

that would give you 30 days and you could extend that by 30 more IF you needed to at the immigration office for 1900baht.

Sadly there is no option C :/