Because this argument is both most delicate and (I believe) most important with respect to students of color, here is one version of a spiel I’ve given in private conference [39with certain black students who were (a) bright and inquisitive and (b) deficient in what U.S. higher education considers written English facility:

   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 SBE 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.

Tense Present, David Foster Wallace