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