On April 23, 1996, at 1015 central daylight time, a Cessna 152, N46748, registered to a private owner, and operated by Texas International School of Aviation, of Tyler, Texas, as a Title 14 CFR Part 91 flight, was substantially damaged during a forced landing near Bullard, Texas. The certificated flight instructor and his student were not injured. Visual meteorological conditions prevailed for the local dual instructional flight. The flight originated at Pounds Field, Tyler, about 30 minutes prior to the accident. The flight instructor said that the student pilot was executing touch and go landings to runway 30 at Tarrant Field, in Bullard, Texas. At approximately 600 feet AGL on the downwind leg he heard "the engine pop and the RPM dropped to approximately 2,000 RPM." Within 10 to 15 seconds the engine power further decayed to 1,700 RPM and "altitude could no longer be maintained." The instructor elected to attempt a downwind landing on runway 12. The airplane touched down past the mid field point of the 2,700 foot grass runway. Braking on the wet grass was minimal and the airplane overran the runway, went through two fences and nosed over, coming to rest in the inverted position. Examination of the airplane revealed that both wings sustained structural damage. Examination of the engine by the operator's mechanic revealed that an exhaust valve had failed.