Yes the teachers should teach the grammar of standard English. And I'm sure the teacher concerned does know the *correct* grammar and teaches it to the kids. That doesn't mean, however, that they should never use the local dialect - inside the classroom or out of it.

I mean I can just imagine in a meeting with parents that I might say something like "If y's (as I would say it) have any problems, then just give us a ring". Would that make me unfit to teach at primary level?

Bagpuss