In all my years of sitting in classrooms as a student, 17, i never had a teacher who didn't speak my language. And the few that had thick accents were difficult to listen to. I had to rely on the books more. So I can sympathize with Thai students. But why on Earth FFS do they make us use NES books? The easiest thing to fix are the books. Hello!! Bilingual books in all the book stores. Why do the schools insist on using NES books when obviously the students are not?
Right?! And try teaching Trig to students who completely missed the English boat. Yet I know of at least one expensive school who keeps taking parent's money while also paying tutors on the side.
The only way this 67 year old American woman could keep teaching science in an expensive English program is her fluency in Thai. The parents are paying more for less.