Well, in defence, I should say that most English teachers have spent an awful lot of years studying English literature and, regardless of how smart or dumb they are as people, probably know a lot more about that than someone who didn't.
And I think maybe 99% of English teachers are trying to get you to think and learn, not evangelize our own personal interpretations of a text. I mean, who cares if you agree with me that Invitation is a Gnostic novel or not? The important part is whether you present your alternative case well and, thus, have understood/learned it.
It doesn't matter if the author actually meant that the night was dark or not. The reason you're reading that text in class is because it's well-suited to helping you develop your perceptual and interpretive skills.