If the subject being taught is standard english then it should be taught correctly. Seems a bit of a waste of time to do it any other way. Bagpuss under your system how would your brother spell, how would he speak in another part of the country with a different dialect, or does he have to copy that to make himself understood.

At school (light years ago), the only dialect we ever had to get to grips with was when studying the works of Robert Burns. Our teacher did emphasise that he only ever used it in writing for entertainment, his letters were written in what was accepted as correct english of the day.

To encourage the use of dialect in formal education only serves as a dumbing down exercise, that will ultimately be to the detriment of the pupil.