Since it is a coming of age story, there are going to be many lessons learned in To Kill a Mockingbird, mainly by Scout and Jem.
One of the first lessons Scout learns is empathy. Her father tells her she has to learn to walk around inside other people’s skins, and she applies this lesson first of all to her conflict with Miss Caroline.
Miss Caroline had learned several things herself. She had learned not to hand something to a Cunningham, for one thing, but if Walter and I had put ourselves in her shoes we'd have seen it was an honest mistake on her part. We could not expect her to learn all Maycomb's ways in one day, and we could not hold her responsible when she knew no better. (ch 3)
Scout learns that Miss Caroline had acted out of ignorance, not meanness, and Scout should feel a little sorry for her because she is in a new place with different customs.
Jem also learns a lesson. He learns a lesson in courage from Mrs. Dubose. Atticus tells him that he wanted Jem to spend time with the old suffering woman so that he could learn about “real courage” from her battle to die without morphine.
You rarely win, but sometimes you do. Mrs. Dubose won, all ninety-eight pounds of her. According to her views, she died beholden to nothing and nobody. She was the bravest person I ever knew. (ch 11)
Jem realizes that courage is not always physical. It comes in different forms, and in unexpected places.
The people of Maycomb learn a lesson about racism. Atticus reminds the men of the jury and the people in the courtroom about their responsibility as a people to make decisions based on what is legal and right and not race.
"I'm no idealist to believe firmly in the integrity of our courts and in the jury system-that is no ideal to me, it is a living, working reality. Gentlemen, a court is no better than each man of you sitting before me on this jury. A court is only as sound as its jury, and a jury is only as sound as the men who make it up. I am confident that you gentlemen will review without passion the evidence you have heard, come to a decision, and restore this defendant to his family. In the name of God, do your duty." (ch 20)
Unfortunately, the jury does not acquit Tom Robinson. They do deliberate longer than expected, showing that some of Atticus’s lessons about race got through.