In Thailand and many other countries, there is not a set standard as to which "style" of English is used.
This is not necessarily a bad thing; I see value in students having exposure to all variants of English, because in reality they may encounter speakers of many different English flavors (or flavours).
The problem my students face, though, is getting confused when encountering, for example, British English in one course with one teacher, and the next semester encountering American English with another teacher; it is hard enough to understand one way of speaking and writing, much less understanding that there are differences in what is considered correct, especially when the Thai language is essentially uniform (save a few local dialects) throughout all Thai speakers.
My approach has always been, whenever possible, to point-out potential variants of any English topic I'm teaching, making sure to stress that all of these variants are correct. I do this because my students typically come from mixed backgrounds and levels, and seldom have been taught the same grammatical structure of this or pronunciation of that.
In my eyes, as long as the students can communicate in this second language, I should not nit-pick accent or other details except at very advanced levels. Of course, this is unless I am teaching a course specifically for a British or American-based test (hello IELTS and TOEFL), in which case the students are essentially paying to learn a certain style, presumably to study or live in a country reflecting that style.
The problem then becomes other teachers I've encountered; often, I've heard teachers admonish students for spelling something "colour" instead of "color," or pronouncing "skedule" instead of "shedule." To me, this not only damages the purpose of [specifically] ESL learning, but also creates a situation of placing one variant of English as more superior than another. It becomes iPhone versus Android; PC versus Mac.
In my eyes, this competition is pointless and counterproductive; English, especially as a second language, should be a medium with which to communicate, not a medium with which to judge what is correct and what is not. I always tell students to simply pick a style they feel comfortable using and stick with it, so I always find it hard to accept colleagues who insist on using American or British English in a classroom outside these countries without at least explaining why this is being done.
That said, schools in Thailand are just as guilty of using accents and styles to discriminate against teachers. I can't tell you how many times I've heard "The students can't understand his accent" from a Thai teacher about a foreign teacher, no matter where he or she is from.
Toning accents down
I'll grant that some English accents are especially "thick" and difficult to understand, and that teachers should strive to speak clearly no matter what the accent; I had to tone-down my southern US accent as soon as I moved here to be understood (and so as not to be compared to George Bush...), as many other teachers from other parts of the world also do.
That said, picking a teacher solely based on his/her accent and/or nationality ends up being counterproductive to the students.
If students only have exposure to Australian accents, for example, and then meet someone from South Africa, how are they supposed to cope? What about needing to speak to any other non-native English person in English? Especially for Thai students who are not typically taught to think in context, I can guarantee that a student who learned only with American or British speakers, native or not, is going to have a very hard time understanding someone who speaks with... well, any other accent.
The bottom line, I tell my students, is to speak to be understood. English isn't just spoken by native English speakers, after all, and no one accent or style is "correct." Why compete?
I hope you enjoyed my blog. If you would like to get in touch or perhaps e-mail me with a question, I would love to hear from you - All the best, Sam Thompson.