In "To Kill a Mockingbird" why did Jem and Scout not leave the school until almost everyone else had gone?

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!