First, Scout sees social injustice up close when Calpurnia takes her to her black AME church. Scout realizes that many of the members of the church can't read. She also finds out that, because her husband is in jail, Mrs. Robinson is being shunned by the white community and can't find work as a maid. The black community, though very poor, has to pull together to raise money so her family can survive. It is unjust that Mrs. Robinson is being held accountable for her husband's actions, especially when he has not even yet been found guilty of a crime.
Second, Aunt Alexandra comes to live with the Finch family during these chapters. She represents the conventional white thinking that leads to social injustice in Maycomb. She considers people superior or inferior based on their ancestry, not on their merit, and ancestry also helps her condemn blacks as inferior (after all, what have the ancestors of the black population accomplished but being slaves?). She is, as well, part of the Missionary Society, which is more concerned with helping blacks in Africa than needy black people in their own community. This helps maintain the inequality in her community.
Third, the white community's condemnation of Atticus for mounting a real defense for Tom Robinson shows social injustice at work. The white townspeople are more interested in upholding the racial line in the community, in which the word of any white person is trusted more than the word of any black person, than finding out what really happened between Mayella and Tom.