Bokurano: Ours

Fifteen middle school students are happily attending a summer camp by the sea, until one of the children leads the group into a cave that will drastically alter the course of their lives forever. Inside are several computers, along with a man named Kokopelli who offers the students a chance to play a game: one where they use a gigantic robot to defeat enemies who attack the Earth. The children agree to sign a contract by touching a shield-like object, and Kokopelli transports them to the cockpit of the Zearth—the mecha they are to pilot.

After an enemy mecha rises from the sea, Kokopelli shows them how to mentally control the Zearth, and defeats it. The children are excited by the prospect of defending the Earth, but not everything is as it seems. The Zearth is in fact powered by life energy, and at the end of every battle, the pilots die. After signing their lives away, the children must now use their time left on Earth to defend it.