In the book Roll of Thunder, Hear My Cry, what happens in Strawberry and how does Casse get revenge?I need to know exactly what happened and how.

In Chapter 5 of Mildred Taylor's Rolling
Thunder, Hear My Cry, T. J. , Cassie, and Big Ma load into the wagon and ride
all the way to Strawberry, Mississippi.  There, Big Ma pulls the wagon containing milk
and eggs into a line far from the front.  When Cassie asks why they do not move the
wagon closer in line, Big Ma tells her that line is for whites only.  When the crowd
thins by noon, Big Ma pulls the wagon out and parks in front of  a building where a
shingle hangs that reads, "Wade W. Jamison, Attorney at
Law." 

As they wait in the wagon, T. J. suggests that they
run down to the Barnett Mercantile while Big Ma is busy.  After they arrive at the
store, T. J. weasels his way into the store and hands Mr. Barnett a list.  While Mr.
Barnett fills this list a white woman comes in and Barnett drops everything and waits
upon her.  Then, as Barnett returns to help the children, a white girl gets her order
filled. Having a girl be waited on before them infuriates Cassies, so she reminds Mr.
Barnett that he has been waiting on them.  However, Mr. Barnett harshly rebuffs Cassie,
telling her to get over to where she was and just wait.  Nevertheless, Cassie declares,
"We been waiting ...nearly an hour...And it ain't fair."  This comment infuriates Mr.
Barnett, who screams "Whose little n---is this!"  A woman from another wagon tries to
quiet Cassie, but she yells instead.  Mr. Barnett tells Stacy to get her out of the
store.  Outside, Cassie asks Stacy, "What's the matter with you?"  He swallows his
anger, and he replies that while they know what they are, Mr. Barnett is not sure of
what he is.

As Cassie sulks, she runs into Lillian Jean
Simms.  When Lillian Jean demands an apology, Cassie says that she is sorry, but she
will not obey the order to get down in the road.  Just then Mr. Simms appears and
demands that Cassie apologize to Miss Lillian.  Cassie says, "I-I apoligiized already." 
Then, Big Ma appears and tells Cassie that they are going home, but not before Mr. Simms
demands that Casie apologize.  Cassie is forced to apologize.  After   apologizing,
Cassie fleescryint and scrambles into the back of the wagon.  Cassie
narrates,

When the words had been spoken, I turned
and fled crying into the back of the wagon.  No day in all my life had ever been as
cruel as this onw."