Shadow Children Series 

The Shadow Children series is a seven book series released between 1998 and 2006 by Margaret Peterson Haddix. The books are: Among the Hidden, Among the Imposters, Among the Betrayed, Among the Barons, Among the Brave, Among the Enemy, and Among the Free. The premise of the series is that the characters live in a dystopian society in which third children are banned (AKA shadow children). It's intense and I cruised through this series as a kid.