English has never consisted of only one dialect. The Angles, Saxons, and Jutes who invaded Britannia in the Fifth Century AD spoke a number of mutually intelligible dialects, all forms of what we call Ingvaeonic, or North Sea, West Germanic. The Frisians also spoke a dialect from this speech continuum. After the establishment of various small kingdoms, each kingdom’s people spoke their own local dialect. Just before the Viking invasions, the four most prominent dialects of Old English were West Saxon, Kentish, Mercian, and Northumbrian, centered in the kingdoms of Wessex, Kent, Mercia, and Northumbria.
The Vikings took half of the Old English kingdoms’ land out of the middle, and almost took the whole. Their elites spoke Old Norse, which had a profound impact upon the English spoken by their new subjects. The conquest of the Norse kingdoms and the unification of what came to be called Angelcynn, then finally Englalond, took place under Alfred the Great and his descendants. Alfred’s grandson Athelstan was the first king of a united English nation.
The language of the king of this new, united realm was from Wessex. Alfred and his successors also sponsored a literary revival of English as a means of unifying the realm. So the dominant dialect of Old English, in which the vast majority of surviving manuscripts are written, was West Saxon Old English. This was the “standard” English of its day.
That day ended shortly after the conquest of England by William of Normandy. In a few years, almost the entire English nobility and senior clergy had been replaced by newcomers who spoke Norman French. This didn’t mean that English was no longer spoken, but only that nobody who was anybody spoke or wrote in English.
English continued to be spoken by ordinary people, of course. And English continued to change, as it always has. Already at the end of the Old English period, certain changes were evident from Alfred’s day. The impact of Old Norse upon English was now evident. After 1100, we call the language that people were speaking Middle English. And as the Frenchified elites grew up speaking English (and had to learn French in school), eventually literary works began to be written in English again. Several different dialects of Middle English – descendants of the various dialects of Old English – were current, but with the increasing commercial and royal importance of London, the East Midlands dialect of London became more and more important. This is the language that Chaucer wrote in. His English, the language of business and government, became the “standard” English of his period.
English continued to change, and by the Tudor period, we begin to talk about Early Modern English. This is the language of Shakespeare and the King James Bible. These two sources, especially, define the “standard” English of the period. For years afterward, everybody knew them and tried to imitate them.
Modern English dates from about the time of our Constitution. And from this time, competing Standard Englishes begin to appear. American thinkers of a nationalist bent began to attempt to modify the spelling and pronunciation of English for Americans. Noah Webster, with his dictionary and spellers, began to define American “standard” English.
While America has fewer dialects of English than Britain does, there are still several distinctive ways of using English in America. What we call American Standard English does not align perfectly with any of these regional speeches; nevertheless, when we talk about “correct” English usage (and assign grades based upon its mastery), this particular dialect (ASE) is the one we call “correct” or “standard.” We teach it to schoolchildren all over the country not as a fossil, but as a ticket to advancement. The more one has the mastery of ASE, the more opportunities one will have for employment beyond the old neighborhood. “Standard” English is the language of business, government, education, and publishing. The better your English (by this criterion), the more you can mix with and influence others who circulate in those arenas. Not only that, but if you move from one place to another, from one dialect region to another, you can still use ASE to communicate wherever you go.
What we call “standard” English at any given time is the language of business, government, education, and publishing. If you were living in 1040, we would be teaching you West Saxon Old English. If you were living in 1400, we would be teaching you East Midlands Middle English. If you were living in 1640, we would be teaching you the language of Shakespeare and the KJV. But since you are living in modern America, we teach you the language current across the all the leading elements of our society.
However you speak and write at home is fine – for home. We don’t need to teach you how to do that. But when you go to school, we will teach you American Standard English – the “standard” English of our day and our society – and mark you “incorrect” for every non-standard usage. That’s the teacher’s job. If you were learning German, we would be teaching you Standard German, not Swiss German or Bavarian (though we might note some differences appearing in those dialects). But we would want you to be easily understood, and to garner respect, wherever you used your German. (I attended a church service in Switzerland a few years ago. Most of the liturgy and hymns were in Swiss German, but when the pastor began to preach, he switched to Standard German. I understood much more of his sermon than I did of the rest of the service.)
So when the teacher corrects your English, that’s not a put-down. You are not less of a person for the way you speak or write at home. But it’s the teacher’s job to teach you this particular dialect, and your job to master it. What you do with it later is up to you.