Without further information it is a little difficult to directly answer your question with a certain "yes" or "no." Instead, I will give you some advice and hopefully you can apply it to your essay in a way that will help you write as well as possible.
First, I think it sounds perfectly acceptable to assume that as a fictional character, Scout is as true to life as Harper Lee paints her to be (which is by all means, a very realistic portrayal of someone who could very well be a real person). In your essay, assuming Scout is a real person works if you are comparing her to other people, making note of real human emotions and showing her actions and reactions in light of true-to-life and true-to-history circumstances. She is certainly portrayed as a child who is somewhat extraordinary as compared to other children, but given that the book is told from her adult perspective, it can be assumed that much of her childish "wisdom" has been added, with humor, from an adult perspective on past events.
I caution you not to talk about the novel itself as a true story (nor Scout as a real historical figure) but it is okay to note that events and attitudes portrayed in the story are based on actual historical events and attitudes. And though Harper Lee likely included some auto-biographical details to this story, I also caution you not to confuse the narrator of the story with the author Harper Lee. Basically, in your essay, you want to be clear to designate that you are writing about Scout, the narrator, who grew up to tell a story of her childhood that has a very powerful message, and base your evidence off of details in the text. In this way, it is okay to write from a perspective which assumes that, though fictional, Scout as a character is very realistic.