Among the Enemy (Shadow Children #6) by Margaret Peterson Haddix

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?