Roll of Thunder, Hear My Cry by Mildred Taylor might be an interesting comparison. It has a number of simiarities to Harper Lee's novel: it is set in the Deep South in the early 1930's, it involves a young girl who learns some unpleasant realities regarding race relations in her community and world as she is growing up, and it is historically accurate and a good example to of how one can learn from historical fiction. Both Harper Lee and Mildred Taylor are from the Deep South that they wrote about; however, Taylor's parents, who were African-American, left Mississippi and moved North when she was small to avoid the racial conflict that was so pervasive there. Thus, Mockingbird and Roll of Thunder examine very similar issues from two very different points of view, a white Southern woman, and a black Southern woman respectively.
Both authors created novels that evoke both laughter and tears, and neither can be faulted for creating feel-good endings that are unrealistic. Both protagonists, Scout and Cassie, begin their stories with the immaturity, idealism, and naivete that should be part of everyone's childhood--and both girls learn some heartbreaking lessons by the time the novels reach their resolutions.
There are two titles that come to mind when I think of possible comparisons to To Kill A Mockingbird, particularly when I think of books that are often assigned and read in school.
The first is Ender's Game as another story of a young person growing and learning how to fit into their world and trying to understand a more complex set of morals than the one that they are brought up with. Just like Scout, Ender has to build from a very strict and clear sense of morals that he has from his short and intense childhood to quickly build a far more complex set of morals in a difficult and intense world at Battle School. Of course the setting and the action are completely different but the themes give an opportunity for comparison.
The second is Catcher in the Rye, and again though the setting is rather different, the way that Holden tries to make sense of the world around him with the guidance (or in his case mostly without any guidance because he can't accept it) of the adults around him. He struggles mightily because he doesn't have a parent like Atticus or adults like Calpurnia and others that help to steer Scout in the right way and to understand the things in the world that don't make sense to her.