Can I obtain a re-entry permit with a 6 month ED visa in Thailand?

Hello, I applied for a 6 month ed visa, the date they told me to come back for my stamp is the date I was supposed to fly to Cambodia. I thought I seen in the mountain of paperwork I had to sign, that if I leave, I lose my visa. I started reading the past posts on re-entry permits but did not see a post regarding 6 month ed visas and re-entry permits(please forgive me if I missed it). Can I get a re-entry permit on a 6 month ed visa? If so, can I apply for it at immigration when I get my stamp? Thank you all, especially Todd and Robert for all you do for the community.