My friend had a serious talking to when we came back from VN. He was on an ED visa for a few months. IO asked him to speak the alphabet and asked him questions in Thai. That didn't go so well. So off to the little room in back. After his Thai gf talked to the IO, they let him but made a notation about his visa. His 50th BD was in a week and we went for a retirement visa.
So it seems your actually have to be a decent student to maintain the ED visa
I'm heading over there after the holiday to actually talk with the bank. I've heard a lot of conflicting stories. Done say it has to stay in a separate account for 4 months. Others say it stays in a frozen account for 4 months after they issue the letter for immigration. Others say it's a combination of 4 months before and after.
One person said an entire year in a frozen account.