This is the second book in the Shadow Children series by Margaret Peterson Haddix. I read it pretty quickly, so that could be seen as an indication of how good I thought the book was!
Luke has now assumed the identity of Lee Grant, in order to come out of hiding as a shadow child (any third child, since any children more than two are not permitted in this vision of the future). He's in a very odd boarding school without windows, trying to figure out what he's supposed to be doing and how to survive in this new world he's been thrust into. And most of all, he's trying to figure out who, if anyone, he should trust.
This was a good sequel to Among the Hidden, and I highly recommend it to anyone who likes futuristic sci fi.