Are you planning on returning to Thailand someday?
What *MAY* happen, if you do not cancel an extension of stay before leaving Thailand, is you will be charged the maximum overstay fine when you apply for a new extension of stay. That fine is 20K THB.
If you are not going to return to Thailand, then you won't get fined.
Yep, that needs to be cancelled. You will go to the immigration office, with a letter from the school saying you no longer study there. They will cancel the extension of stay. IF you're not leaving the same day, you will apply for another extension of stay - the 60 day extension of stay based on unable to travel due to COVID-19.