Why does Calpurnia arrive at the courthouse in Chapter 20 of To Kill a Mockingbird?

At the end of Chapter 20, Calpurnia makes her way to Atticus, who has just given his closing remarks. Calpurnia politely asks Judge Taylor if she could pass a note to Atticus. Calpurnia delivers a note written by Aunt Alexandra, which says the children have been missing since noon. Mr. Underwood tells Atticus that Jem, Scout, and Dill are sitting in the Colored balcony, and they've been there since noon. Atticus then calls for Jem to come down, and the children climb across Reverend Sykes to make their way toward Atticus. Scout mentions that Calpurnia looked "peeved," but Atticus appeared to be exhausted. Atticus tells the children to return home with Calpurnia, which Jem protests. Atticus allows the children to return to hear the verdict after they eat their supper. Calpurnia then walks the children home and yells at Jem for taking Scout to see the trial. 

Why does Calpurnia come to the courthouse in chapter 21 of To Kill a Mockingbird?

Calpurnia arrives at the courthouse with a message for Atticus that his children are missing. This is somewhat comical as the children are both in the courthouse at that moment, avidly watching and listening to Tom Robinson's trial. Calpurnia's entrance also occurs at a moment of high tension when Atticus has just finished summing up, urging the jury to do their duty and acquit Tom Robinson. This has the effect of defusing the tension somewhat at that particular moment.

Atticus was totally unaware up to this point that Jem and Scout have been following the trial so closely. As he has been powerless to stop them, he says they might just as well follow the whole thing through and hear the verdict as well when it is delivered. Having heard all the evidence which to any reasonable mind would appear to be heavily weighted in favour of Tom, Jem is confident that Tom will be acquitted. However, the opposite happens, simply because Tom is  black and discriminated against, and Jem is gravely disillusioned. Tom Robinson's trial is his and Scout's first major experience of adult prejudice and injustice, and it is an absolute eye-opener for them. Atticus, it seems, would have preferred them not to attend the trial, but he couldn't have hidden the outcome of it from them in any event. 

Why does Calpurnia come to the courthouse in chapter 21 of To Kill a Mockingbird?

In Chapter 21, Calpurnia has arrived at the courthouse to give Atticus a note that tell him that Jem and Scout are missing and have not been home since just after one in the afternoon. The children are found in the colored balcony watching the trial. Atticus says the children need to go home to eat, but he says that they can return to hear the verdict.

How did Calpurnia learn to read in To Kill a Mockingbird?

Calpurnia learned how to read when she was at Finch's landing. A Miss Buford taught her to read using the Bible and a primer.

One Sunday when Atticus is called to Montgomery, the children go to church with Calpurnia. While there they experience what it is like for the poor blacks of Maycomb: The church is without a ceiling and painted walls; pine benches are set out in rows instead of the pews to which the children are familiar. There are no pianos or organs, no church programs, no hymn-books. Zeebo, Calpurnia's son, leads the congregation in the hymns by first reading the line and the others follow, singing the line.

After the service, the children ask Calpurnia about this technique of "lining." Then, Jem remarks that perhaps the congregation could save the collection money for a year and purchase some hymn books. When she hears this, Calpurnia laughs: "Wouldn't do any good...They can't read." Scout and Jem are shocked, but Calpurnia says she is one of only four there who can read."Where'd you go to school, Cal?" asked Jem.

"Nowhere. Let's see now, who taught me my letters? It was Miss Maudie Atkinson's aunt, old Miss Buford--" (Ch.12)

Further, Calpurnia explains that there was no school for the children to attend, so she taught her son Zeebo herself. Scout asks if she taught him "out of a primer, like us?" Calpurnia says that she made Zeebo read a page from the Bible every day, and then read from a book from which Miss Buford taught her.  "...bet you don't know where I got it," she said. The children do not know. Calpurnia said, "Your Granddaddy Finch gave it to me." (Ch. 12)

Calpurnia explains that she grew up between the Buford Place and the Landing. She adds that she worked for both the Finches and the Bufords, and she moved to Maycomb with Atticus and their mother after they married.

This history is not unlike those of others like Calpurnia who grew up on one family's land where they were cared for in other ways than what was necessary for servants. The interest in Calpurnia's education was not uncharacteristic of these families. Later, when Atticus talks to his sister Alexandra, he alludes to the bonds of his and Calpurnia's youth when he says that Calpurnia is a part of his family.

In To Kill a Mockingbird, how did Calpurnia teach her eldest son, Zeebo, to read?

In Chapter 12, Calpurnia takes the children to her church, and Lee gives the audience a signficant view of Cal's culture and background.  Jem and Scout are in awe after their experience and ask Calpurnia about Zeebo's ability to read (since only 4 people in the church can read).  Calpurnia tells them,

"I made [Zeebo] get a page of the Bible every day, and there was a book Miss Buford taught em out of" (Lee 125).

The other book is Blackstone's Commentaries, a book about law (not too ironic since Calpurnia got it from the Finches).

Who taught Calpurnia to read in To Kill a Mockingbird?

Calpurnia grew up near Finch's Landing where she worked for Atticus's father before moving with Atticus to Maycomb after he was married. She also learned to read and write there, and she is one of the few members of the First Purchase Church who can do so. Cal also educated her son, Zeebo, from the same book--"Blackstone's Commentaries"--from which Atticus, Jem and Scout learned. Calpurnia also lived near Dr. Buford--Miss Maudie's father (she is actually a widow and her maiden name was Buford)--and while working for the Bufords, it was Miss Maudie's aunt, "old Miss Buford," who taught Cal to read.