At an unknown time, a Cessna 172P was substantially damaged at an unknown location and by an unknown pilot. The Chief Flight Instructor from the operator reported that the aircraft is used for rental purposes and had been flown several times since the last maintenance inspection. On about September 4 or 5, a pilot reported that during the pre-flight inspection, he noted wrinkles in the skin on a wing. A mechanic looked at the aircraft and reported that it was okay for flight. On September 7, the aircraft went in for the scheduled 100-hour inspection. During this inspection, maintenance personnel reported the wing spar and the horizontal stabilizer spar were both bent as if they were overstressed.