In what chapter does Cecil Jacobs jump out of the dark and scare Jem and Scout?

Cecil Jacobs jumps out and scares Scout on Halloween night in Chapter 28.

Cecil Jacobs is a neighbor of Scout and Jem.  He is sort of a friend of theirs, because they live on the same street and are around the same age.

Cecil Jacobs, who lived at the far end of our street next door to the post office, walked a total of one mile per school day to avoid the Radley Place and old Mrs. Henry Lafayette Dubose. (Ch. 4)

Scout and Cecil sometimes fight.  She wallops him in the schoolyard one day because he made an announcement to all of the kids at school that her father “defended niggers.”  Scout does not really understand what the big deal is, but she knows it’s an insult.

For the Maycomb County pageant on Halloween, Cecil Jacobs is dressed as a cow and Scout is a ham.  The purpose of the pageant is to showcase the agricultural wealth of Maycomb County.  That night, Cecil rides to the school with his parents and then comes out in the dark to find Scout and Jem.

A circle of light burst in our faces, and Cecil Jacobs jumped in glee behind it. “Haa-a, gotcha!” he shrieked. “Thought you’d be comin‘ along this way!”

“What are you doin‘ way out here by yourself, boy? Ain’t you scared of Boo Radley?” (Ch. 28)

The children are not afraid of Boo Radley at this point.  He is their friend, and they have had a few clandestine communications like notes on Boo's porch, gifts left in a tree and a blanket on Scout's shoulders.

Because of this incident, when the children hear someone following them later they assume it is Cecil trying to scare them.  It is actually Bob Ewell trying to kill them.  Boo Radley sees what is going on and comes out to rescue them, killing Bob Ewell.  Jem gets his arm broken in the struggle, but Scout comes out relatively unharmed.  Boo goes back to their house with them, and Scout gets to walk him home later.