Without getting too dictatorial, it is a teacher's duty to teach the language that is most useful and standard English is readily understood by all speakers. That doesn't mean colloquial language is out of bounds, as colloquial language is a vibrant part of standard English. However, students should also be exposed to the varieties of English, which they will struggle with otherwise when they come across a non-standard speaker.
I'm coming from an ESL perspective, but with native speakers, too, standard English is the most useful. In these cases, I would recommend their learning Englishes, which isn't that difficult- I speak one English in the pub and another in the class.