The flight instructor said that he was giving an end-of-course stage check to the private licensed student, who was taking a commercial pilot course. The instructor said that the flight was normal and upon returning to the airport he requested to see a short-field landing. They landed in a nose-high attitude and there was "only a slight bump” when the aircraft’s main wheels touched down. Two more landings were performed by the student, which completed the stage check. Four hours later the flight instructor took the same student out for another lesson. The airplane had not been flown during this 4-hour period. During the airplane’s preflight examination, he found a large crack in the left wing's surface immediately above the landing gear attachment point. The flight instructor stated that the student did not make any hard landings during the first flight of the day. The student stated that she did not remember anything unusual about the flight.