I don't know anything about teaching. I know a fair bit about being taught, in both private and public schools.
I don't disagree teaching 40+ students would be a hard row to hoe, and would tempt many teachers to just go through the motions.
I was fortunate in having good teachers at the primary and secondary stage, who encouraged and extended me, to the point I won scholarships at both the secondary and tertiary level. The first WAS in a 1950's classroom.
OTOH, I had a history teacher who made the subject insufferably dull, although I now find it fascinating.
I don't know how private and public schools compare in Thailand. IME, private school teachers in the West are better paid, have manageable class sizes, and are better at what they do. Perhaps it is a positive feedback loop.