The three previous posts all provide excellent responses to your question. I, too, agree with them that Atticus provides constant guidance and support for his children as they ascend toward adulthood. But Jem and Scout learn most of their important lessons from the experiences garnered from the people with whom they meet and associate. In addition to the previously mentioned Dolphus Raymond, Calpurnia, Tom Robinson and Miss Maudie, the children learn about life from other characters as well.
- They discover that their single-parent world isn't so bad after seeing how Dill's parents treat him.
- They recognize the complexities of a person's nature from the actions of B. B. Underwood.
- They see that the well-educated do not always make the most intelligent decisions (Miss Caroline and Miss Gates).
- They learn about different types of courage from Mrs. Dubose.
- They see that law enforcement officials (Heck Tate) can dispense their own kind of justice when others (the all-white jury) fail to do so.
- They learn to recognize the positive and negative nature of family pride through their own Aunt Alexandra as well as from Mr. Radley.
- They discover that people's reputations are not always what they seem (Boo Radley).
What an interesting question! I'd agree with my colleagues, of course, that Atticus--by words and example--teaches Jem and Scout most of the lessons they learn in the course of this novel. I'd also make the case, though, that others were responsible for some "life lesson teaching," as well.
Dolphus Raymond showed the kids (including Dill) it's possible to live beyond the stereotypes. He's still having to do a ridiculous thing like let thewhite world think he's a drunkard, but through him they're able to see beyond that to the reality that color is relatively unimportant to living a contented life--and the unnfairness of having to hide in order to be content.
Miss Maudie demonstrates, in word and deed, what it means to be a true believer and follower of Christ in thae face of hypocrisy (in the form of the "foot-washin' Baptists"). She also helps them discover that their view of Atticus as a feeble old man who can't do what other "cool" dads do is flawed.
Calpurnia takes them to church and is able to show Jem and Scout a world they really don't understand much about. They had assumed all other Negroes were educated (could read), but they discovered that's not true--yet they are perfectly able to worship together through linin'. They also encounter Lula, a Negro woman who treats them with prejudice, so they're able to see this ugly attitude is a choice, not something inherent or inborn or specific to any one race.
While Atticus is their primary teacher, as he should be, others do contribute to their learning experiences.
As far as we see in the book (a period of about 2 years) we could certainly conclude that Atticus is the most positively influential PERSON in his children's lives. But Atticus does not really provide the life lessons. He simply provides guidance for answers to the circumstances that life presents to the children.
They are growing up in a very racially charged atmosphere, during a time in our history (and certainly in a location) that was on the cusp of major change. Maycomb and its inhabitants, traditions, prejudices, and general daily habits provide the opportunities for lessons. Then, Atticus, as a role model of integrity first, and then a father and lawyer and leader, teaches his children how to look at these situations from different angles and guides them to THINK through things. He doens't give direct answers - ever - but facilitates the thinking process required to gain true understanding and ultimately, wisdom.
The book clearly displays what clairewait has answered. What teaches Scout and Jem the most are the experiences they have as they grow through the events of the story. Their exposure to the heavier things in life, the struggles and battles that Atticus faces, as well as the struggles and battles that come about as a result of his decision to defend Tom Robinson, these are the things that teach Scout and Jem the most important lessons. No one sits them down and says "This is how life is and this is what is most important." Rather it is what Scout observes, how she handles the situations, and what she experiences that give her the wisdom and insight to learn what is most important. As clairewait said, Atticus provides insight along the way and guides his children toward the right way of thinking, both by his example and by his words of wisdom. He gives his children the tools by which they learn what are the most important lessons in life.