Teachers of English grammar are engaged in a prescriptivist enterprise. However, Yates and Kenkel believe it is not difficult to demonstrate the irrationality of the justifications of prescriptive rules, and, by extension, the apparent irrationality of those who adhere to them.

Linguistic tolerance: all varieties of English have equal value and merit respect and appreciation of their special beauty and expressiveness.

The goal of teachers of English grammar should be that all students consciously know the most important principles of Standard English (which, of course, need to be identified) so that they can have the needed linguistic security to decide for themselves when their language use generally, and their writing particularly, should conform to the norms of Standard English, when the norms have no relevance, and when they can consciously decide to violate them. [source]

More grammar notes here.