Discuss the racial and socio-economic issues that arise in the trial scene. Pay attention not only to Tom Robinson, but the Ewells as well.

The trial of Tom Robinson for the rape and assault of Mayella Ewell lays bare the many racial and socio-economic divisions at the heart of Maycomb life. Although Tom's officially on trial for a specific criminal offense, in actual fact his "crime" is that he's an African American male who's transgressed the established norms concerning inter-racial relations.
On the witness stand, Tom makes the biggest mistake of his life when he admits to feeling sorry for Mayella. White folks at court are aghast upon hearing this. Though nothing more than an expression of basic humanity, Tom's concern for Mayella is immediately construed as an offense against the unwritten code that determines relations between the races. Expressing pity for Mayella places her in a subordinate position to someone regarded as her racial inferior. This turns the whole system of race relations upside-down and is therefore perceived as a threat to the dominant white race.
As well as having a distinct racial hierarchy, Maycomb is also home to long-standing class divisions. Even in a society based on the domination of the white race, not all white people are equal. The Ewell family certainly isn't. They're widely despised throughout the town as "white trash," and thought of as the undeserving poor who won't work for a living and whose appalling personal behavior puts them beyond the bounds of respectable society.
Bob Ewell, patriarch of the clan, is totally humiliated by Atticus on the witness stand. He thought he would be a big hero in Maycomb for saving his daughter's honor against the sexual predations of a savage black man. Yet his conduct on the witness stand further exposes him as an ignorant, stupid man, which serves to reinforce everyone's negative opinion of him. Atticus knows that his is a hopeless brief, but if he can play upon the social prejudice of the townsfolk towards "white trash" like Bob Ewell, then he figures he might be able to at least chip away at the credibility of the prosecution's case.