What should I do about my education visa before leaving Thailand on October 26?

Always good that people ask to be sure, but every 90 days you went with the school to Immigration and signed the STM2 form, see example.

Show this to the school and maybe they will ask Immigration to understand what it means to inform Immigration when the reason for the stamp change. a Change is ending education in your case.

I recently did the visa cancellation of my ED visa before leaving Thailand (apparently this is the correct way). It took about 3 hours at main Chiangmai immigration and I needed a letter from the school as well as copies of passport page, all stamps, all 90 day reports and TM30.

You say "𝗢𝘂𝗿 𝗳𝗶𝗻𝗮𝗹 𝘀𝘁𝗮𝗺𝗽 𝘀𝗮𝘆𝘀 𝘂𝗻𝘁𝗶𝗹 𝗢𝗰𝘁𝗼𝗯𝗲𝗿 𝟮𝟲" <- this leads me to believe that your current (and final) stamp also says "extension of stay permitted until" and has that Oct 26th date?

If that's the case, no matter what the school says you DO need to cancel that ED visa extension BEFORE you exit the country. (in fact some borders won't even let you stamp OUT on an 'open' ED extension)

Get documentation from your school stating you stop studying on the 26th of Oct, GO to the immigration office where you got that extension about a week before that date. They should cancel the extension and give you a new stamp allowing you to stay until the 26th. Then you can just stamp out