School's Out - Forever (Maximum Ride, Book 2) by James Patterson
This book has less of a distinct storyline than the first book, but more action and drama. There is even a little romance. With the content rating - It's an action book so there is a lot of fighting. And there is more swearing in this book than in the first.
The flock's identity is discovered, and they are offered refuge by a high ranking FBI Agent. All of the kids have a chance to go to school and live among the non mutant population for the first time in their lives. Unfortunately, someone close to the kids is a traitor, and their peaceful existence is in jeopardy. Max must discover the traitor and help her flock find the meaning of a real family, before they become the school's lab rats, permanently.