Speaking frankly, the general level of English ability in Thailand ranges from very poor to very much worse than that.
Of course there are exceptions, and I suspect that is due mainly to some exceptional teachers out there working exceptionally hard often to get only exceptionally average results.
Nevertheless, those really English language classrooms are quite rare. There is a reason why so many Thais laugh at this YouTube clip, and it is not because the skit is so outrageously unrealistic.
Don’t stress just because most of the skit is in Thai, you can easily follow what is happening without understanding the dialogue. In some ways not comprehending the student mutterings even adds an extra dimension to what is going on. I hope you enjoy it.
Oh, the YouTube video is available here.