The Unwanteds series is a group of fantasy books written for ages 8-15 by Lisa McMann that quickly rocketed to the New York Times bestselling list, and for good reason. Book four (out of the planned seven) titled Island of Legends is a fantastic contribution to the series that your middle grade kiddos won't want to miss.
At the beginning of the series we meet Alex Stowe. Every year in Quill (where he lives) there is a Purge where all of the thirteen-year-olds are sorted into three categories: the strong and intelligent ones are Wanteds and go to the university. Others are sent into the Necessities (where they end up working for the Wanteds), and the final group, typically the creative and artistic kids or Unwanteds are sent to their graves. Alex is sent to his death while is brother, Aaron is sent to the university. But instead of dying, Alex and his fellow Unwanteds end up in a magical place where they learn secrets of their world.
A lot has happened since then, and in the fourth installment Alex's magical skills are improving and he is growing more confident by the day in Artime. Aaron's leadership is threatened back in Quill, and more creatures are discovered that can both help and harm these two different teams.