as soon as you leave without a re-entry permit you visa expires. You can get a re-entry permit at the airport for 1000 baht if your visa is multiple entry.
My son says you may need to leave the country to cancel your Ed Visa but again he's not 100% sure. Its a pity there is no immigration office in Chiang Rai to ask them there as it can change at different locations. I'm sorry I can't be of more help.