The simple answer is it is fiction. The storytelling of Harper Lee is often thought to be semi-autobiographical. The fact that she places the story in the 1930s during a time when the race relations of the south had not progressed past the theme of segregation make this story appear to be very realistic. I think you are safe in calling it "realistic fiction." I would reference in a book report the fact that many of the characters in the book seem to be based on actual people from Harper Lee's childhood memories. She has stated many times that it is not autobiographical. The argument continues.
The first answer is what you want, but I always like to add that Harper Lee calls it a love story. My students always call me crazy when I agree with her. It is a novel about all kinds of love: for a father between his children, for a neighbor (Boo) for whom he considers "his children," and for the decent people in Maycomb county for their own people--whether black or white. It truly is a love story.