The Roar by Emma Clayton

The Wall was built to keep the animals out after they were infected with The Plague, or so Mika had been told. Mika had suspected his world may be built on secrets ever since his twin sister, Ellie, vanished a year ago. Mika takes the chance to search for his sister when a mysterious organization starts recruiting mutant kids to compete in a violent virtual reality game, Pod Fighter. As the voices in Mika's head build to a deafening roar, he'll discover the secrets behind The Wall and disclose the lies he and his parents have been told all their lives.