@Wira ********
they don't convert anything, they are here on a 30 day visa exempt entry OR a 60 day tourist visa entry and then they apply for the Non-Immigrant visa at the immigration office IF their school can supply the documentation. Not every school and not every immigration office can do it.
Most places give you paperwork to leave the country and get a 90 day single entry Non-ED visa at a thai consulate.