You can cancel in Bangkok. But they will want proof from your school possibly. I cancelled my non-b in another province. Then applied for a new visa the same day.
Haha, this is funny. Just went to CM immigration, you need your school to cancel your work permit. You need 1) letter from school saying you are done, 2) both forms form labour office saying your work permit is cancelled. Then you can go to the immigration office they will cancel your business extension one year and you can then on the same day apply for the covid extensions (same day as WP cancellation). Letter and forms don't have to be original just scanned and email to you