Is it appropriate to elect a variety of English language as the only correct one to teach?

No

Explanation:

as my English professor said 'the English language is fairy used upon the globe", so if the majority of the population is pronouncing a specific word wrong then the pronunciation of the word is correct because most of the people are using it, so you must teach the English that is used in your community so you're students may understand. for example I am from did UK and when I came to America no one understood my way of speaking English. even though it's the right way of speaking in the UK. they called it gibberish. people told me I talk to fast. I've then went to college and studied English.