When a teacher asks an high school student to write an essay assessing whether a work is a successful example of horror fiction, the assignment has two purposes. The first is to determine whether the student read the story carefully, and the second is to help the student develop a sense of genre. Thus there are two things you should do in writing your essay.
First, as you read through the story, highlight moments that seem most to give the impression of horror. What makes them frightening? How does Poe create an atmosphere of pyshological suspense? Most particularly, what are the characteristics of the narrator that heighten the effects of the story?
Next, for the sense of genre, what other works have you read in school or outside which create a sense of horror? Do they do so more or less effectively than Poe? How and why?