To elaborate on some of the above, the most profound passage on social inequality is found in Atticus's closing remarks to the jury in Chapter 20 of Harper Lee's To Kill a Mockingbird.
In this passage, Atticus dispels the ideal belief that "all men are created equal" as Thomas Jefferson wrote. Atticus argues that the belief is just an ideal, an ideal that is often misused to do such foolish things as "promote the stupid and idle along with the industrious" in public schools. Instead, contrary to what many people would like to believe, Atticus argues the following:
We know all men are not created equal in the sense some people would have us believe--some people are smarter than others, some people have more opportunity because they're born with it, some men make more than others, some ladies make better cakes than others--some people are born gifted beyond the normal scope of most men.
We can find many examples throughout the book that fit Atticus's description of social inequality. One example is seen in Burris Ewell and the rest of the Ewell children. In Chapter 3, we're informed that Burris Ewell has been attending the "first day o' the first grade fer three year now" and the rest of the Ewell children are scattered in grades all over the school even though all Ewell children never attend more than one day of school out of the year. Yet, just as Atticus says, in the name of equality, rather than all the Ewell children continuing to be placed in the first grade, they keep getting promoted though they haven't earned the grade.
In addition, Atticus himself exemplifies his point of some people being born with more opportunities than others. Atticus is the son of a wealthy farmer, and the prosperous estate has been in the Finch family for generations. Because Atticus was born of wealthy parentage, he had the opportunity to decide to be the first male Finch not to inherit the farm and instead pursue a law degree, enabling him to become a highly successful, valued lawyer and state legislator. In contrast, others who were not born in his position will never be granted the same opportunities. For example, Walter Cunningham Jr. is destined to work his family's farm all his life, whereas Tom Robinson's wife and children are destined work the low-paying labor jobs offered to African Americans in this time period.
Many of the characters in Harper Lee's To Kill a Mockingbird suffer from various forms of social bias. Certainly, Boo Radley is a prime example. His mental instability is dealt with by shutting him away in the family home, and he is blamed for any abnormality that occurs in the town. All of Maycomb's African-American population face racial biases of some sort, and Tom Robinson faces harsher treatment because of his supposed connection with the white Mayella Ewell. Dolphus Raymond, who prefers the company of African-Americans, is considered mentally unstable because of this decision. The poor citizens of Maycomb face different inequities. The Cunningham and Ewell families, though dirt poor, are treated differently; the Cunninghams are poor but honest, while the Ewells are considered "trash." Women also face unequal status in Depression era Alabama. They are not allowed to participate on juries, few of them are employed, and most of them are presented in a quirky view by the author.