According to the flight instructor, he and the student pilot were on the second leg of a cross country flight when the student pilot entered the downwind leg of the traffic pattern at an intermediate airport. The flight instructor directed the student pilot to perform a power-off landing, and power was reduced to idle when the airplane was abeam the runway numbers. The airplane slowed to glide speed, and the student pilot selected 10 degrees of flaps and turned it to the base leg of the traffic pattern. After turning onto the final leg of the traffic pattern, the airplane's airspeed decreased to below 60 knots, and the flight instructor advised the student pilot to perform a go around. The flight instructor then took control of the airplane to prevent an "imminent stall" and applied full power, but the airplane continued to descend. The airplane struck trees and a fence, then impacted the ground. The flight instructor subsequently reduced the power to idle and applied the brakes. However, the airplane struck another fence, turned 180 degrees, and came to rest in a grassy area to the left side of the runway. The airplane sustained substantial damage to the wings and fuselage during the accident sequence. The flight instructor did not report any preexisting mechanical anomalies with the airplane.