In my estimation To Kill a Mockingbird is a well-structured novel, paced nicely with brief episodes that have independent a beginning, middle and end of their own while also contributing to a larger development of action, character and theme.
The moral and moralizing do seem to fit the O'Connor criticism cited by vangoghfan above. (post #6)
After reading some of the work linked to in post #6, I would agree that the novel's aesthetic structure is certainly sound and also entertaining. Yet, I still have some reservations about the overt nature of the moral, the symbolic nature of the Atticus character (while acknowledging the value message he serves), and the convenient access the children seem to have to subtle emotional and intellectual truths that are not available to other characters in the novel.
I disagree with the view that this is, on the whole, a realistic novel. It is a moral tale concerned with real issues, expressing its moral quite directly and baldly and using its characters as vehicles of instruction. It's a good book and it draws in high quality influences, but it is not without its flaws.
Because it is so entertaining, it makes a natural selection for use in literature courses for high school students.