Among the Barons (Shadow Children #4) by Margaret Peterson Haddix
Luke Garner, an illegal third child, spent his first twelve years in hiding. For the past four months Luke has lived among others, using the identity of Lee Grant, at the Hendricks School for Boys. But just as things are finally starting to go right, Lee's little brother Smits arrives at the school and Luke finds himself caught in a tangle of lies that gets more complex with every passing day.
Can Luke trust Smits to keep his secret? And can he trust Smits's menacing bodyguard, Oscar?