Another advantage of arguing that Miss Emily is a symbol of the post-Civil War South is that you will be able to find some good evidence in the story to support this claim. For example, in the first paragraph, the narrator states that, when Emily died, the men in the town went to her funeral out of "a sort of respectful affection for a fallen monument." Monuments are often symbolic—of a particular person's bravery or accomplishments, of a particular group's dedication and service, and so forth. Further, the narrator says,

Alive, Miss Emily had been a tradition, a duty, and a care; a sort of hereditary obligation upon the town.

In this way, she is described metaphorically again. You might find that it is especially fruitful to explore the ways in which Emily can be considered a "tradition" in the town. Her house, for example, is traditional, and it stands all alone, "lifting its stubborn and coquettish decay above the cotton wagons and the gasoline pumps—an eyesore among eyesores." Emily and her home seem to be quite connected: both are out-of-date and idiosyncratic and odd. She's even received special treatment as a result of everything she embodies, like when Colonel Sartoris invented a story about her family not owing any taxes or when Judge Stevens grew irritated with someone complaining about the smell of Emily's house. He asks, "'will you accuse a lady to her face of smelling bad?'" As "a lady," especially a lady who is symbolic of a certain set of traditions and social mores and etiquette, Emily enjoys certain benefits.

Choosing the stance of Miss Emily is a symbol of the post-Civil-War South is a solid beginning for a high school essay. It’s a position most critics would agree with. The main advantage is that it agrees with what we know about Faulkner and the story. There are two main disadvantages. For a stance to be something you need to prove, it has to be something that is in doubt, i.e. that your readers might disagree with, or find new or original. Since the position is one generally accepted, it fails to meet the criteria of something that needs to be argued. The second problem with it is that it is too general. It’s a good starting point, but you need to narrow it down to some specific aspect of the post-Civil War South. Does she represent something good or bad? Is she a victim as much as an oppressor? You have a good starting point, but now you need to focus your topic more narrowly.