NARIT will provide accident and health insurance from the time you arrive at Immigration in Thailand up until the time you pass through Immigration again on your way home.  Note that this insurance policy does not cover the period of travel before you reach Immigration in Thailand, or after you leave Immigration on your way home.  Nor does it cover such things as loss of money or luggage, or changes to flight bookings that result from circumstances beyond your control, so you may wish to take out separate travel insurance yourself before coming to the school.