It's commonly understood corruption is an acceptable daily part of Thai life, and schools are no exception.
I worked on and off for 10 years at several branches of the largest private school system in Southeast Asia. The advice given to all new farang teachers from existing farang teachers: "We're not here to teach, we're here to give the appearance of teaching." Of course that was a little tongue in cheek, but there was a lot of truth to that statement.
The examples of corruption ran strong with that school, from the directors on down. If they can avoid paying a promotion or any other bonus to a returning teacher, well it's just cheaper to let that teacher go and get another fresh victim.