@Tod ********
agree with this from my recent personal experience. A few months ago, under ED visa, I went for a trip to Vietnam and I was strongly questioned at the back. I had the letter from school, the whole questioning was in Thai (I was studying Thai language) and they let me in after 15 minutes of questioning.
Please, note: I went for a quick trip to Vietnam over the Christmas. THREE days over a weekend, while all the schools in the country were closed for holidays. I mean, it was crystal clear that I wasn't missing any class because it was national holidays period... and still got questioned.
My advice: do NOT travel out of Thailand under an ED, not even for one day.