As for segregation, it was the way of the American South (and much of the North) during the 1930s and well beyond. It may seem unusual now, but separate schools, restaurants, restrooms, churches, hotels and neigbhorhoods for blacks and whites were the norm at the time of the story.
Social injustice was alive in TKAM just as it is in many ways today. African-Americans were considered inferior by many white people; Boo's mental state relegated him to an outcast; the Cunninghams are outsiders because of their poor financial situation and their distant homes in Old Sarum; and no one will have anything to do with the Ewells because of their past (and present) reputation.
The three children--Jem, Scout and Dill--experience a coming of age during the first half of the novel, primarily through their contact (though minimal) with Boo. They learn that Atticus has a secret or two up his sleeve, and Jem discovers a new meaning of courage. Their loss of innocence only grows in Part Two.