One of the major contrasts in part 2 is that Jem turns 12 and tries to act more like an adult, and distance himself from Scout.
Part 2 begins with the announcement that Jem has turned 12. As a new 12-year-old, Jem considers himself an adult. He wants to tell Scout what to do now, and this annoys her to no end.
Overnight, it seemed, Jem had acquired an alien set of values and was trying to impose them on me: several times he went so far as to tell me what to do. (ch 12)
Although Jem’s attempts to separate himself from Scout and childish ways both her immensely, they also signal a shift in the book’s focus. Scout and Jem grow up quickly in part 2. They have to finally face the full storm of the trial, racism, and the prejudice that goes along with Southern small-town life. The book becomes more about Tom Robinson and Atticus, and less about Boo Radley and the kids. The reader has been introduced to the town with a slow build-up, and now everything happens at once, full force.