@Alex ******
and that you did that and got away with it after you came back and tried to get an extension is great for you
BUT the real way to do it IS
if you are on an extension of stay based on education you should cancel it before you leave the country because just stamping out doesn't cancel it,
AND
many MANY people have had issues returning to thailand, and then trying to get a visa or extension inside the country because they had an uncancelled ED visa extension in their passport