Everything written is ok so far, but let me add something:
- I warmly recommend to go to the local immigration for the re-entry, better don’t wait to be at the airport
- some immigrations don’t accept students in ED Visa being out for more than 14 days; it’s not by the law, but don’t forget that they are “free” to decide who stays out. On samui, for instance, up to 7 days out of the country it’s ok, up to 14 only for specific reasons and with a leave of absence letter. For more than 14 days out of the country they might deny the entrance with the same visa
You can go to the immigration, get 7 more days with 1900thb. This is the safest and most legal solution. Overstaying is dangerous and anyway will cost you 2000.
I just confirmed that yes, it's possible to extend another 7 days the current 30 days extension, paying 1900thb and then no more extension will be possible on that visa.
Immigration will be closed on Monday for HM the Queen's Birthday.