Just want to second everything Johanna said. Of course teachers at all levels need to teach standard English (and here in Quebec standard French), but this can be done without denigrating the pupils' or students' language. I quite like the idea of the before and after tape-recording, but just use different terms like "formal" and "informal" or "our local" versus "national or international" and the children will see themselves as they are: bi-dialectal. Mieke