The False Prince Book by Jennifer A. Nielsen

In the fantasy novel The False Prince, a sarcastic and rebellious orphan named Sage is kidnapped by Conner, a corrupt nobleman aiming to avert a civil war after the secret murder of the royal family. Conner forces Sage and two other orphans into a cutthroat, two-week competition to train as the long-lost Prince Jaron, planning to crown the winner as his puppet king and execute the losers. Sage relies on his sharp wits and defiance to win the deadly game, leading to a massive climax where it is revealed that Sage is actually the real Prince Jaron in hiding. Having used Conner's treasonous plot to safely infiltrate his own palace, Jaron brilliantly exposes the traitors, arrests Conner, and reclaims his rightful throne.