As an English teacher (from the South of the UK), I have a "Standard" accent, yet I realise I use features of non-Standard English. For example, today a (linguist) friend noticed I said, "is anyone sat here?" Instead of "is anyone sitting here?" In my opinion, it is more important to be authentic but also to notice that there is variation in language around the "Standard". It's good to draw our students' attention to this variation. Yes they should learn standard forms and the "proper" structures. But they should also recognise and be introduced to regional variation - how will their "Standard English" knowledge help them in real life situations? They will be lost in northern England, or Scotland, for example. Students are not stupid. You can point out these features to them, or they can notice when you use "non-standard" features, and you can have a discussion about it. I automatically say "yeah" and colloquial language in the classroom. It is authentic and the kind of language they'll be exposed to when they watch TV or when they talk to native speakers. It is therefore important to use and teach these forms. At the same time, we can point out the register and when it is appropriate to use them. You can simply tell them not to use yeah or hiya in formal writing. End of story.