What are 3 quotes that support the thesis "social classes play a major role in human interaction in Harper Lee's To Kill A Mockingbird"?

Jem has a really great quote that I think that you could use.  The quote is found in chapter 23.  Jem says the following lines to Scout.  

"There’s four kinds of folks in the world. There’s the ordinary kind like us and the neighbors, there’s the kind like the Cunninghams out in the woods, the kind like the Ewells down at the dump, and the Negroes.”

The quote illustrates a bit of racism, which you are not going for based on your thesis, but the rest of the quote should work nicely.  The quote shows that Jem sees the people of the world divided into four distinct social classes.  People like his family, poor but honest people, poor and dishonest people, and Negro people.  The order that Jem provided is interesting to note as well.  His list is a ranked hierarchy and moves from "best" to "worst."  

The next two quotes you could use to further explain the difference between the Ewell and Cunningham social classes.  

“You, Miss Scout Finch, are of the common folk. You must obey the law.” He said that the Ewells were members of an exclusive society made up of Ewells. In certain circumstances the common folk judiciously allowed them certain privileges by the simple method of becoming blind to some of the Ewells’ activities. They didn’t have to go to school, for one thing. Another thing, Mr. Bob Ewell, Burris’s father, was permitted to hunt and trap out of season.

The Ewells are members of the poor class that don't do anything to help themselves out of being poor.  They skip school and break laws.  That's contrary to the Cunninghams who are poor, but honest, hard working, law abiding members of society.  

 I thought I had made things sufficiently clear. It was clear enough to the rest of us: Walter Cunningham was sitting there lying his head off. He didn’t forget his lunch, he didn’t have any. He had none today nor would he have any tomorrow or the next day. He had probably never seen three quarters together at the same time in his life. 

 I tried again: “Walter’s one of the Cunninghams, Miss Caroline.” 

 “I beg your pardon, Jean Louise?” 

“That’s okay, ma’am, you’ll get to know all the county folks after a while. The Cunninghams never took anything they can’t pay back—no church baskets and no scrip stamps. They never took anything off of anybody, they get along on what they have. They don’t have much, but they get along on it.”