Tomorrow Girls: Behind the Gates by Eva Gray (Published by Scholastic, May 1, 2011)
In the near future, life in the United States has changed as a result of a series of natural disasters as well as a war that has gone on for as long as thirteen-year-old Louisa can remember. Many wealthy parents are sending their children to boarding schools in the wilderness in hopes of keeping them safe. Louisa's parents are wealthy doctors, and they have arranged for her to attend a boarding school named Country Manner School with her best friend, Maddie, by giving Maddie a new identity as Louisa's twin sister.
At CMS, Louisa and Maddie are roommates with two other girls, Rosie and Evelyn. Louisa and Rosie both love being at CMS, enjoying the fresh air and survival skills classes after growing up in a polluted city. But Maddie is unhappy. She misses her real family and hates the rules of the school. Evelyn doesn't like CMS, but for a different reason. She is suspicious about CMS and doesn't believe the adults there who tell them all the rules are for their own safety.
I think younger readers interested in the dystopian genre but not quite ready for older young adult books will enjoy this book. However, I think that the background of the story was underdeveloped. There's pretty much nothing in the book at all about what is going on out there besides the fact that there were some really bad natural disasters and now the United States is involved in a war against "the Alliance," but there's never any explanation of what exactly the Alliance is, or why there is a war. There are going to be three more books in the series so hopefully there will be some answers in the other books.