In chapter 11-15 everything and everyone are different. More than half of the population had died, in hiding, or left small town Pennsylvania in attempt to get a better life. Things were supposed to be better in Texas power, food and shelter, most everyone packed up and left. The small part of the population that was left tried to make the best of things. School started back and things seemed pretty stable for the time being.
In the novel Life As We Knew It chapter 11-15 things did not start off as planed. "Our informal survey showed 16 Freshmen, 7 Sophomores, 4 Juniors, and 6 Seniors." Pg 130 pdf. The informal survey showed that the high school student population decreased by more than 50%. Students had been kidnaped, died, or moved. The option of homeschooling was available about 6 student (including Miranda) left. One of the emerging themes from chapter 11-15 was the population downsize.
The incident with the moon forced people to make irrational decisions. Everyone was motivated to do something and in a hurry. Miranda's mom had to make decisions that would have the best outcome for her family, so did everyone else. Actions and decisions that were made had negative outcomes for everyone, for instance the hospital only accepting a select few, the school serving food, etc. With everyone making irrational decisions for themselves affected everyone else.