The flight instructor and student pilot were practicing a soft field takeoff procedure. During the takeoff roll, the airplane began a yaw to the right. The airplane continued to the right after the main landing gear wheels departed the runway, and the flight instructor told the student to correct back to the center of the runway. The airplane then yawed to the left past runway heading and the pitch attitude increased. The flight instructor attempted to take control of the airplane; however, the student did not let go of the controls. The flight instructor tried to reduce the pitch attitude, but the airplane stalled and impacted terrain adjacent to the runway. The airplane sustained substantial damage to the fuselage. The flight instructor reported no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation.