Shonk notes this wonderful passage from David Foster Wallace discussing what I think is just the right way to approach to dealing with students who speak, say, Black English or Brooklynese instead of 'Standard Written English': You let them know their is nothing wrong or inferior about their dialect, but that in universities and in the career they probably want after school, a different dialect is used, and they must learn it:
I don't know whether anybody's told you this or not, but when you're in a college English class you're basically studying a foreign dialect. This dialect is called 'Standard Written English. ... From talking with you and reading your essays, I've concluded that your own primary dialect is [one of three variants of [Standard Black English] common to our region]. Now, let me spell something out in my official Teacher-voice: The SBE you're fluent in is different from SWE in all kinds of important ways. Some of these differences are grammatical — for example, double negatives are OK in Standard Black English but not in SWE, and SBE and SWE conjugate certain verbs in totally different ways. Other differences have more to do with style — for instance, Standard Written English tends to use a lot more subordinate clauses in the early parts of sentences, and it sets off most of these early subordinates with commas, and, under SWE rules, writing that doesn't do this is "choppy." There are tons of differences like that. How much of this stuff do you already know?
[STANDARD RESPONSE: some variation on "I know from the grades and comments on my papers that English profs don't think I'm a good writer."]
Well, I've got good news and bad news. There are some otherwise smart English profs who aren't very aware that there are real dialects of English other than SWE, so when they're reading your papers they'll put, like, "Incorrect conjugation" or "Comma needed" instead of "SWE conjugates this verb differently" or "SWE calls for a comma here." That's the good news — it's not that you're a bad writer, it's that you haven't learned the special rules of the dialect they want you to write in. Maybe that's not such good news, that they were grading you down for mistakes in a foreign language you didn't even know was a foreign language. That they won't let you write in SBE. Maybe it seems unfair. If it does, you're not going to like this news: I'm not going to let you write in SBE either. In my class, you have to learn and write in SWE. If you want to study your own dialect and its rules and history and how it's different from SWE, fine — there are some great books by scholars of Black English, and I'll help you find some and talk about them with you if you want. But that will be outside class. In class — in my English class — you will have to master and write in Standard Written English, which we might just as well call "Standard White English," because it was developed by white people and is used by white people, especially educated, powerful white people.
[RESPONSES by this point vary too widely to standardize.]
I'm respecting you enough here to give you what I believe is the straight truth. In this country, SWE is perceived as the dialect of education and intelligence and power and prestige, and anybody of any race, ethnicity, religion, or gender who wants to succeed in American culture has got to be able to use SWE. This is How It Is. You can be glad about it or sad about it or deeply pissed off. You can believe it's racist and unjust and decide right here and now to spend every waking minute of your adult life arguing against it, and maybe you should, but I'll tell you something: If you ever want those arguments to get listened to and taken seriously, you're going to have to communicate them in SWE, because SWE is the dialect our country uses to talk to itself. African Americans who've become successful and important in U.S. culture know this; that's why King's and X's and Jackson's speeches are in SWE, and why Morrison's and Angelou's and Baldwin's and Wideman's and West's books are full of totally ass-kicking SWE, and why black judges and politicians and journalists and doctors and teachers communicate professionally in SWE. Some of these people grew up in homes and communities where SWE was the native dialect, and these black people had it much easier in school, but the ones who didn't grow up with SWE realized at some point that they had to learn it and become able to write in it, and so they did. And [INSERT NAME HERE], you're going to learn to use it, too, because I am going to make you.