Shadow Children #6: Among the Enemy book by Margaret Peterson Haddix

HIDE OR FIGHT?Matthias, an illegal third child, is caught in the cross fire between rebels and the Population Police. When he unwittingly saves a Population Police officer, Matthias is brought to Population Police headquarters to train as an officer himself. There he meets Nina, another third-born who enlists his help in a plot to undermine the Population Police. But Matthias is under constant scrutiny, and he has no idea whom he can trust. What can one boy do against a wicked bureaucracy?