In Harper Lee's To Kill a Mockingbird, the grouping of chapters would probably look like this:
Chapters 1-7 deal with family, school and Dill: this includes Boo Radley escapades.
Chapters 8-12 take the reader into Part II of the novel. This section includes Miss Maudie's fire, the first snow Scout sees, Christmas with Francis and Uncle Jack, and Tim Johnson. Also included is Jem's time with Mrs. Dubose and the kids going to Calpurnia's church. There is a great deal in this section: it is almost like a string of short stories.
Chapter 13 starts with the arrival of Aunt Alexandra who will stay at the Finch home for an extended period of time. There is family strife over this; Dill runs away to Maycomb, a mob shows up at the jail where Tom Robinson is staying and Atticus is keeping watch; and the court case begins. The reader gets clear insight into the Ewell family, especially Bob and Mayella. The court case comes to a close at the end of Chap. 21.
The last section starts with Chap. 22 and goes on until the novel is over. This includes Jem trying to deal with the court's decision, with the fate of Tom Robinson, Aunt Alexandra and Scout becoming united in purpose, and the fall pageant taking place. It is during the closing section of the book that Scout and Jem fight for their lives, and Boo Radley finally comes out of the house.
If you are looking to compress portions, I would do so with the family history at the start, but perhaps make sure to talk about Maycomb—still reeling from the effects of the Civil War and now the Depression. I would highlight only the major points of the court case. The children attending Calpernia's church could be summarized, as well a good deal of Aunt Alexandra's ideas on how the Finch family works, and the children's knowledge of their heritage.
The rest of the story is extremely rich; the novel's setting may turn some students off—it's as if they are watching a black and white movie. When I sell it, though, I try to stress that it is a story with excitement, mystery, attempted murder, and an unlikely hero.
Hope this helps.