Can I cancel my ED visa extension at an immigration office and get a 7-day extension before leaving Thailand?

Normally you get a document from the school stating you stop studying on the date your extension expires and you go a week or so BEFORE that date to the office where you got the extension.

Immigrations cancels the extension and gives you a stamp good until the date the paper from the school says you stop studying (the date your current extension expires) <- THEN you have time to plan to leave

IF you wait until the last day and cancel it, most of the time they will sell you a 7 day "application for extension not approved" stamp that gives you a week to get out of thailand BUT they don't have to sell that to you.

Nope you can't cancel an extension at the airport OR at an immigration office that didn't issue the extension you're on now.