Into the Fire by Pittacus Lore

After escaping from their sabotaged plane and avoiding a crash, Six, Sam, and their new charges manage to make it to the Human Garde Academy and meet up with Nine. Instead of continuing their investigation, though, the Garde decide they must take on the dangerous new foe who has abducted their friends and allies.

But the trail has gone cold, and the only way to find their enemy is to infiltrate his organization. Against Six and Sam’s better judgement, they send Rena and Nemo undercover. But things take a frightening turn when the two young teens are forced to play a deadly game, and Six and Sam may not be able to reach them in time.