Following two successful stop-and-go landings in the tailwheel-equipped airplane, the student pilot departed for his third solo flight around the airport’s traffic pattern. While on final approach, the student’s flight instructor observed from the ground that the airplane appeared to be high and slow and requested via the aircraft radio that the student execute a go-around. The student pilot complied and while he was on the downwind leg of the traffic pattern, the flight instructor instructed him to make a full-stop landing. During the landing roll, the student pilot retracted the wing flaps and added engine power as if to takeoff. The flight instructor reminded the student that this was to be a full-stop landing. The student pilot then reduced engine power; however, he also released back pressure on the control stick. As the rudder lost its effectiveness, the student pilot lost directional control and the airplane exited the side of the runway. The airplane impacted a ditch and nosed over, resulting in structural damage to both wings. An examination of the airplane following the accident revealed no preimpact anomalies with the airplane’s flight control system.