I think the first time that I read this story I was struck by the setting, which is a key component of every Gothic tale and important in creating the glooming and brooding backdrop for the action of terror to follow. The appearance of both Madeline and Roderick made me fearful, and the terrifying finale made me really shiver with a kind of self-indulgent terror! I really do wonder whether there is some kind of vampiric thing going on between Roderick and Madeline - Poe is careful to describe them in ways that deny life or death completely.
Gothic literature almost always evinces a certain horror out of the reader. The macabre atmosphere in the exposition, the House of Usher as, perhaps, representative of the inhabitants of that house is certainly frightening. So, too, is the unreliability of Poe's narrator as his mind seems to be deteriorating. This uncertainty about the narrator affects the reader's response to this story. Consider your interpretation of the final events. Who effects these: Roderick or the narrator?
Only you can answer this question because only you can say how the story makes you feel. I can tell you what emotions it stirs in me, but that wouldn't help you very much. I'm sure your teacher wants your opinion. And think about it: This is the kind of question that you can't get wrong--unless you just don't answer it, that is. So, unless you're just trying to get out of reading the story, you need to think about your own reaction to the story, even if it is confusion. Give it a shot.
I think most people suspect there is some deep dark secret within the house and the family. I certainly did. But I had read a lot of Poe by the time I read FHOU for the first time. There's always a deep dark secret in Poe.
One thing I think a lot of high school students miss, but as an adult I found it terribly obvious, was the clue about Madeline's condition - cataleptic fits - as soon as I read this (the first time I read the short story) my first thought was, "Oh yeah, she's going to get buried alive. They'll think she's dead and she'll be having one of those fits. Classic Poe."
While a lot of students find this "surprise ending" truly surprising, I'm always surprised at how many don't expect it.