Why does Harper Lee end the first part of "To Kill a Mockingbird" at chapters 10 and 11?

As was mentioned in the previous post, Chapters 10 and 11 are important turning points in the lives of the children and the plot of the novel. Both Jem and Scout learn valuable lessons after spending time with Mrs. Dubose and witnessing their father shoot the rabid dog. Atticus also teaches his children what real courage looks like after he elaborates on Mrs. Dubose's battle with her morphine addiction. The first part of the novel primarily focuses on the children's infatuation with Boo Radley and portrays Atticus giving his children valuable life lessons. The second part of the novel focuses more on Tom Robinson's trial and the children's maturation. In the second part of the story, Jem and Scout witness racial injustice firsthand, which deeply impacts their perspectives on life. Both children also apply their father's previous life lessons and develop into morally upright individuals. Harper Lee chose to end Part I after Chapter 12 in order to shift the plot toward the Tom Robinson trial and to explore the children's moral development. 

I agree with the above post. The episode involving Mrs. Dubose is the last experience Scout and Jem will share as children. Everything after this (particularly the trial) will be observed, felt, and analyzed differently between the two. Scout maintains her innocence, and a major conflict in the later parts of the book will be her attempt to understand why Jem thinks/acts the way he does. Jem, on the other hand, has little time for Scout, and he processes each new development in the trial quite personally.

By ending the first part with the Mrs. Dubose episode, Lee is also introducing an important lesson to the children. Think of it as a last attempt to fortify their defenses before they are forced into situations beyond their years. Prior to this, Atticus often tells Scout to keep her head when times get rough, and Mrs. Dubose seems to be the testing ground before they face the rest of the community. Thus, they have one final instruction in courage and grace before they have to actually practice those values.

In my opinion, part one of the book focuses on the childish world of the Scout, Jem, and Dill. Their main focus and concern is with playing children’s games such as the make believe plays about the Radley’s.

The second part of the book focuses more on their exposure to an adult world as the novel shifts its focus from the Radleys to the Tom Robinson trial. The children are exposed to real world problems and lose interest in the childlike world of make believe. We later see they are not immune from the dangers of the adult world as they are thrown into a world of hatred and racism. .

Why does Harper Lee end the first part of To Kill a Mockingbird at Chapters 11 and 12?

To Kill a Mockingbird proceeds through the development of Scout and Jem. In the first part of the book, there is a fascination with Boo Radley, all very innocent. This part of the book represents childhood. The world of adults does not break into this idyllic world. Dill, another child, is very much a part of this world and represents what it means to be a child.

In chapter 8, we begin to see a change. The unnatural winter and snowfall can be seen as an ominous sign, an adynaton. More importantly, Miss Maudie’s house burns down. We, therefore begin to see slowly the world of adults come into play.

By the time we come to chapter 11 and 12, the trial of Tom Robinson emerges. This means that the world of adults is here. Racism, rape, and the like are introduced. Scout and Jem will have to mature quicker on account of circumstances that they are not in control of. This sense of maturity is written clearly in the opening words of chapter 12.

Jem was twelve. He was difficult to live with, inconsistent, moody. His appetite was appalling, and he told me so many times to stop pestering him I consulted Atticus: “Reckon he’s got a tapeworm?” Atticus said no, Jem was growing. I must be patient with him and disturb him as little as possible. This change in Jem had come about in a matter of weeks.

It seems like a fairly logical decision by author Harper Lee to divide To Kill a Mockingbird between the 11th and 12th chapters of her novel. The first seven chapters concern the children's fascination with Boo Radley, the Radley House, and their attempts to get a glimpse of the "malevolent phantom." Chapters 8-11 serve as a transition; Aunt Alexandra's family is introduced in detail, Atticus's secret talent is unveiled, and Jem's march into adulthood begins with the death of Mrs. Dubose. Part Two deals with the second major plot, the trial of Tom Robinson, and the children's trip to Calpurnia's church followed by Alexandra's arrival in Chapters 12 and 13 sets the stage for the trial. Some critics have suggested that Miss Lee should have added a third part following the trial, perhaps beginning somewhere between Chapters 22-26, since the final chapters tie the two plots together when Boo finally makes his appearance to defend the children against Bob Ewell following the Halloween pageant.