Linguistically, of course, Ulster Scots is a dialect of English, as Scots is, too. But what teachers call "English" at any given time is also a dialect within the continuum of the English language. British English (BBC-speak), a.k.a. Oxonion English, or the Queen's English, is one dialect, heavily reinforced by authority. American English, a.k.a. Standard English, is another such dialect, heavily reinforced by authority. Both are native dialects for some people, but learned dialects for many others. They help maintain communication across vast populations and keep us together. But there are lots of Englishes, and there always have been.
The English language was never a unified, single language. From the beginning, even as it diverged from other West Germanic languages, it existed as a cluster of mutually-intelligible dialects. The Angles, Saxons, Jutes, and Frisians sometimes thought of the others as speaking different languages, but they all found they could communicate fairly easily with each other. In any case, there were several regional varieties in Britain even before the Danish invasions -- as there still are.
West Saxon English is thought of as "standard" Old English, but achieved that distinction by historical accident. It was the language of the kings of Wessex, who unified England and reconquered the Danelaw. It was the language of Alfred the Great's scholarly revival. So, it's not surprising that the greatest number of our texts in Old English are in the West Saxon dialect. But WS found itself left behind after the Norman Conquest, since the new rulers conducted business in Norman French. The variety of English that re-emerged as the language of the post-Norman English-speaking court c. 1400 was East Midlands Middle English, a variety of the Anglish (not Saxon) tongue spoken in Mercia -- especially London. And still there were different varieties.
Scots is a variety of Northumbrian, the northern Anglish variety of Old English. After the Vikings conquered York, the speakers of Northumbrian found themselves cut off politically from other speakers of Old English. Their speech endured in the Lowlands and Borders. When the Gaelic-speaking kings who ruled from Perth and were buried on Iona were replaced by the Bruces and Stuarts -- English-speaking Lowlanders -- the Scottish court began speaking the Lowland dialect of Northumbrian English, which is now called Scots. This language was taken over to Ulster in the days of the Plantation by James VI and I, and the form that endures there is called Ulster Scots.
But then, there's Geordie, the Northumbrian dialect that endures in the Borders to this day. York and Cumbria have related but distinct dialects. The Midlands speak one way, and then there's Mummerzet and Cockney and Scouse (Liverpudlian), not to mention Glaswegian, Irish English, and Welsh English. Meanwhile, in America, we have Canadian English, American English, several varieties of Southern English, and other dialects. Add to these Australian English (Strine), New Zealand English, Indian English -- you get the point.
All of these dialects -- whether favored or unfavored, posh or down-market -- are equally English. The standardized textbook forms act as a sea anchor to keep the dialects from drifting apart into mutual unintelligibility. This also makes them valuable economically, and so reinforces the curricular goal to inculcate proficiency in the standardized forms. But after hours, no community's English is any better or any worse than any other community's English.
The goal of the teacher of English is to help students communicate effectively in both whatever native dialect they speak at home, and in the wider worlds of work and inherited texts. The teacher of English should appreciate the local form of English for what it can do; each has its attractions. But the students should be challenged to master the standardized forms to the best of their ability, because this will widen their experience by that of other English writers across the generations, and also make their social and economic advancement easier in the world of work. Teachers should make clear to students when and whether they are being graded on clear and effective usage within their native dialect, and when they are being graded on clear and effective usage within the standardized form.
We can celebrate the local dialect, but we also owe it to our students to help them master the forms that will help them the most in later life.