Did they let your son change classes?
I feel this way about some people from Scotland and I mean no offense to any Scots out there because you are one of my favorite nationalities on the planet. It's just that certain people from certain parts of that nation are extremely difficult to understand even though I am speaking the same language as them in theory. I think this can be quite detrimental to the development of a child's language abilities.
The restrictions being high here in Da Nang would not work in the other parts of Vietnam because thy would find themselves with a massive shortage of teachers if they were to implement this. Here, basically ever teacher in the country wants to live in Da Nang, so they can get away with it.