Scout has the pleasure of playing the role of a ham at the school pageant. Her job seems simple enough; when Mrs. Merriweather calls out, "Po-ork," Scout will enter from stage left. Mrs. Crenshaw designs Scout's costume by bending and shaping wire to look like a ham. The wire is then covered with brown cloth. The costume comes down to Scout's knees, and she has two holes so she can see where she's going.
When the pageant begins, Scout is in costume and waiting for her cue. However, Mrs. Merriweather proceeds to give a "thirty-minute description of Colonel Maycomb’s exploits." Scout discovers that she can "more or less sit" in her costume and by doing so, she soon falls asleep. Mrs. Merriweather calls out, "Po-ork," to no avail. Mrs. Merriweather gives the cue two more times, but Scout still does not respond. Finally, Scout wakes up and enters the stage.
While her late entrance is amusing to many, Scout is embarrassed by her mistake. She and Jem stay behind and wait for most of the spectators to leave because Scout doesn't want to face the crowd after her mistake.
The passage that you are looking for can be found a couple pages into chapter 28. It is after the pageant, and Scout is mortified that she had fallen asleep and missed her cue to come on stage. She was dressed as a ham, and was supposed to come on stage when Mrs. Merriweather called out "Po-ork!" but since she was asleep, Scout missed that cue, and instead came on at the very end, running to make up for lost time. Everyone in the audience laughed, and laughed. They were just entertained at her tardy entrance, and probably thought it was cute, but Scout was horribly embarrassed. She was so embarrassed that she wanted to wait until everyone left so that she didn't have to face anyone in her shame. The book states,
"not even Jem could make me go through that crowd, and he consented to wait backstage with me until the audience left."
So, they just hung out backstage so that Scout didn't have to face anyone in her embarrassed state. She didn't want anyone making comments or poking fun at her. Someone even offered them a ride, and Jem declined, respecting Scout's wishes. So, they set off for home, in the complete dark, after everyone else has left, which means that as they do walk home, there isn't anyone around to hear what is going to happen next.
I hope that helps a bit; good luck!