The certified flight instructor (CFI) was providing flight instruction to the private pilot, who was training for a commercial pilot certificate. The airplane was in the traffic pattern, abeam the runway, when the flight instructor reduced the engine power, and asked the private pilot to demonstrate a power off, 180-degree accuracy landing. The CFI verbally coached the private pilot throughout the approach, and reminded him to watch the airspeed, which was 3 knots slow. The CFI reported that when the airplane was about 100 feet above the ground, the private pilot increased the flaps from 10 to 25 degrees, and pitched the nose up. The airspeed degraded, and the airplane subsequently landed hard, and sustained wing spar damage. The flight instructor had accumulated about 800 hours of total flight experience. The private pilot had accumulated about 190 hours of total flight experience.