For middle and upper grades (I’ve used it in 7th & 8th), an excellent read is Mildred Taylor’s Roll of Thunder Hear My Cry. Themes are developed through following the experiences that a black landowning family faced in the South in the 1930’s. The children go to a separate school for blacks, with castoff books from the nearby white school. The children learn “who they are” as they grow up. An uncle visits occasionally from Chicago in his big Packard (he has a job in the North). The story provides opportunity to explore the basic problems in human nature (as Peter mentioned) that anyone faces. A big one is the “us” and “others” prejudice that is endemic to every group. Another is basic “growing up pains” events. Still another is exploring your own reactions to others, regardless of how badly you have been treated. The story also illustrates how children can “get along fine” as children, but be acculturated into prejudicial attitudes by their elders.
The book provides opportunity for the teacher to expand upon the “us and them” problem to point out that nationality, race, ethnicity, tribes, denominations, socioeconomic status–any way of “grouping” people poses problems. How we both respect other groups and relate to individuals in those groups in a Christlike way does not happen automatically–even if we are Christians.