@Mark *****************
I would still take documentation from your school and GO cancel that current extension re-entry at the immigration office that issued it
We don't know if it will cause you grief down the road.
People on ed visas/extensions have flown out, came back on free stamps, went out again gotten Non-B visas at an embassy, came back in again wtihout issue but when they went to get an extension they were denied until they got the earlier extension/re-entry officially cancelled,
AND
they were also charged 500baht a day for every day that they didn't cancel that ED extension (up to 20K baht fine)
Just saying is all