The pilot, seated in the left seat, said that the instructor pilot, seated in the right seat, asked her to perform a magneto check inflight. During the magneto check, the pilot inadvertently turned the engine ignition key to the off position resulting in a total loss of engine power. As the pilot turned the key to restart the engine, the head of the ignition key broke off. The instructor pilot transferred the controls to the pilot and attempted to move the ignition key but was also unsuccessful. The instructor pilot took the controls and performed an emergency landing into a school football field. The airplane landed hard and sustained substantial damage to the firewall, both wings, engine mount and fuselage. The pilot reported that there were no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation.