In To Kill a Mockingbird, what are some literary devices used from chapter 12 to chapter 24?

Remember that literary devices include not only figures of speech, but also techniques such as the use of imagery, allusion, and irony. Fortunately, Harper Lee is a master of all of the above, and To Kill a Mockingbird has ample examples of them on practically every page. Chapter 12 is the beginning of the second part of the book. This is when Calpurnia takes Jem and Scout to her church because Atticus is gone. Lula, a member of Cal's church, notices the children coming and is described through metaphorical imagery as follows:

She was bullet-headed with strange almond-shaped eyes, straight nose, and an Indian-bow mouth. (119)

With such visual imagery, the reader can picture the shape of Lula's head, eyes, nose, and mouth by drawing upon words and phrases that they understand in the world around them. For example, bullets and almonds have a similar shape, very much like an Indian's bow. Hence, not only does Lee use imagery and metaphors to describe this character, but she also applies allusion to real-world visual images to get the description across to the reader. Other examples of literary devices can be found in the list below.
Chapter 13 - "As a result the town remained the same size for a hundred years, an island in a patchwork sea of cotton fields and timberland" (131). (This is metaphorical imagery comparing the town to an island in the middle of a "sea," not of water, but of cotton fields.)
Chapter 14 - "Punk, punk, punk, her needle broke the taut circle. She stopped, and pulled the cloth tighter: punk-punk-punk. She was furious" (137). ("Punk" is used as onomatopoeia: auditory imagery.)
Chapter 15 - "We had a week of peace together. After that, little, it seemed. A nightmare was upon us" (144). (This is metaphorical foreshadowing: Future life events surrounding the night before the trial are being referred to as a nightmare.)
Chapter 16 - "'Mr. Cunningham's basically a good man,' he said, 'he just has his blind spots along with the rest of us'" (157). ("Blind spots" are metaphors for not being able to see incorrect beliefs within ourselves.)
Chapter 17 - "The varmints had a lean time of it, for the Ewells gave the dump a thorough gleaning every day, and the fruits of their industry . . . made the plot of ground around the cabin look like the playhouse of an insane child" (170). (This is a simile comparing the Ewell home to a playhouse of an insane child.)
Chapter 18 - "Apparently Mayella's recital had given her confidence, but it was not her father's brash kind: there was something stealthy about hers, like a steady-eyed cat with a twitchy tail" (181). (This is also a simile: Mayella's testimony is compared to a cat while using the word "like".)
Chapter 19 - "Mayella Ewell must have been the loneliest person in the world. She was even lonelier than Boo Radley" (191). (This is an allusion referring to someone that the audience knows: Boo Radley.)
Chapter 20 - "One more thing, gentlemen, before I quit. Thomas Jefferson once said that all men are created equal . . ." (205). (This is an allusion referring to someone that the audience and the jury know: Thomas Jefferson.)
Chapter 21 - "If Mr. Finch don't wear you out, I will--get in that house, sir!" (207). (The dialect here is used to further characterization of Calpurnia.)
Chapter 22 - "'Just looka yonder,' he pointed. 'Every one of 'em oughta be ridin' broomsticks. Aunt Rachel already does'" (217). (This is an allusion to witches.)
Chapter 23 - "Jem, see if you can stand in Bob Ewell's shoes a minute" (218). (This is a metaphorical idiom.)
Chapter 24 - "Mrs. Merriweather played her voice like an organ; every word she said received its full measure" (230). (This is a simile comparing the woman's voice to singing and reading music.)

This sounds like a daunting question, but Harper Lee’s language is so rich that the literary devices are not too difficult to discover. I’ll try to get you started with a couple, and you can continue from there.
Allusion is a literary device used often in To Kill a Mockingbird. This is a reference by the author to another subject. It could be a reference to a literary work or an event in history. It is up to the reader to have the background knowledge to understand the allusion. For example, in chapter 12, the author references the Bible as she talks about the mischief Jem got into one time at church. She says, “until someone investigated and brought forth Eunice Ann saying she didn’t want to play Shadrach any more—Jem Finch said she wouldn’t get burnt if she had enough faith, but it was hot down there.” This refers to the story in the Bible of Shadrach, Meshach, and Abednego and the fiery furnace. The author is alluding to this story.
Another literary device is imagery. The author uses this at the beginning of chapter 13 as she lets us know that Aunt Alexandra is here to stay. “I heard the suitcase hit the bedroom floor with a thump. The sound had a dull permanence about it.” She gives us both the word “thump” and the idea that a sound can be permanent, and these contribute to the reader’s sense that this is no ordinary visit by their aunt.
It might help you to have a list of literary devices handy as you skim the book for them. A helpful website is linked below.
Lee, Harper. To Kill a Mockingbird. (Soft Cover). Perennial, 1961.