Darkest Days — N.W. Harris

The energy-based aliens believe in a system of trial by battle. They seek to push Shane and his friends into the arena with the ones who killed their parents. The results will determine if humans deserve to live, or if they should be made extinct as well. It's up to Shane to keep his friends--and an army of kids who look up to him--alive. They'll be fighting not just for their own lives, but for the fate of the entire human race. Can the enemy of Shane's enemy be his friend, or is this just another species determined to exploit and destroy them?