why does miss caroline cry after the children leave for lunch

When Scout explains that Walter can't pay back the lunch money Miss Caroline offered, the teacher taps Scout's hand with a ruler and makes her stand in the corner of the room. Scout and the children are puzzled by this very unthreatening form of "whipping," and the entire class laughs until a locally-born sixth grade teacher arrives and announces that she'll "burn up everybody" in the room if they aren't quiet.

The first half of the day ends, and on her way out of the classroom, Scout sees Miss Caroline bury her head in her arms as the children leave the room. However, Scout doesn't feel sorry for her considering her unfriendly treatment that morning.