Do you need to cancel your education visa before leaving Thailand?

so get documentation from your school that says you stop studying on the day you fly out

GO To the immigration office that issued that extension the one in Chiang Mai by the airport, about a week before that date and they'll cancel the extension and give you a new stamp letting you stay until the day you fly out.

Then just leave

Depends if you are on a visa or extension of stay. If the latter you need to cancel it. If you are actually on a visa you don't

Your school is full of it, and unless you're NEVER EVER gonna get a new Non-Immigrant type visa (OF ANY category) you need to cancel your ED extension.

If you don't you will still be able to get in and still be able to buy a tourist visa/get in on a tourist visa

but

you could run into issues applying for an extension if they see you have an "open" ed extension in your passport (one you didn't cancel correctly before you left the country)

AND

you could be fined for it as well especially if you're going to get a Non-Immigrant type visa / extension

I sincerely doubt you have an ED VISA and that you stamp really says "extension of stay permitted until" and has a date <- that is not a visa that is an extension and you have to cancel it before you leave

SO

I would say get new documents from your school and go cancel the extension like you're supposed to, because stamping out doesn't cancel it officially nor does letting it run out.