I like this question :)
The generic answer is 1984; but I'm not so sure that's necessarily the best book for teaching English. It is a great book, one for an Ethics or Political Science class, but there are better, more beautiful examples of the English language.
My personal pick is Mary Shelley's original Frankenstein. I read it in grade 11 during the Gothic section of our course, and it really opened my eyes to how powerful literature can be.
I mean, here's a woman (Mary Shelley) who is married to one of the most well-known iconoclasts of the early 19th century (Percy Shelley), and is the daughter of literally one of the first real, powerful feminist figures of the age (Mary Wollstonecraft). She is surrounded by the famous romantic poets of the day, both her husband and Lord Byron and more close associates-- her father is the controversial political philosopher William Godwin. And all this in 19th century Europe where women still don't exactly have great opportunity.. What on earth could she possibly do to stand out??
How about-- write a novel over a fortnight that will captivate the minds of millions of readers for hundreds of years while also really making deep, intelligent commentary about society.
The use of literary devices, such as the obvious metaphors, but also the subtle, underlying expression of nature in the form of pathetic fallacy, make her work literally beautiful. But for me, it's all about what she's saying in the subtext.
You can really see how the most profound ideas of the day-- the science that her husband and his friends would have been privy to, the atheism that was uncommon at the time, and her parents' philosophical views-- really influenced this book.
My final point, since this was my favourite part, is about the satanic-hero (or Byronic-hero) character. The story's similarities to John Milton's Paradise Lost are uncanny-- the monster even reads Paradise Lost and speaks about it in the book-- and this was the first time I was introduced to this type of character. I later discovered how deep and intricate these ideas are when you lay them all out, and I only got there because of reading this one single book.
As one of the first, and best, examples of both Gothic literature and Science fiction, this is my personal pick for which book should be read by all students in highschool English classrooms. Many many many other books would also be acceptable-- as long as we are making them read something!!