But who is to say that the teacher in question was incapable of switching? Maybe it just comes down to a difference in what he/she considered appropriate for that particular meeting and what Mooh considered appropriate.

That's hardly a sacking offence, is it? Again, since I don't know just how informal the US *yous* is, I can't comment on whether everyone would be expected to consider it inappropriate. I just know I wouldn't consider the equivalent in UK English to be so informal that it should never be used in the situation described.

I think we can teach kids standard English and therefore expand their horizons and opportunities without implicitly teaching them that their own dialect (and by extension their social class and community) is somehow inferior. One way to do that is for the teachers to show (by example) that they can use both forms and point the way for when each form is the norm and advantageous.

Bagpuss