Anita Stewart is a social worker at the school, and she is asked to stay late in case of violence at the homecoming football game. Why does she decide against staying?

She feels that there is no way to change the situation, as she would not be able to make things change and the situation would not happen. In this case, she understands that she won't be able to do what she was told because she doesn't have what it takes, or that there is no way for her to do that.

Positive and negative reinforcement indeed serve to strengthen wanted behavior. The difference is that with positive reinforcement you're trying to positively influence and change a certain type of behavior while with negative reinforcement you're trying to extinguish a certain pattern of behavior which isn't desired.