The student was undergoing a flight check with the Chief Flying Instructor (CFI), and was asked to demonstrate a short field approach and landing onto the grass runway. A speed of 65 kts was maintained until the aircraft approached the runway threshold, at which point the stall warning horn sounded intermittently. The student reported that she closed the throttle at about this time and the aircraft landed heavily. The main landing gear oleo struts were heard to "bottom". A further circuit and landing was carried out without incident. The CFI carried out a post-flight inspection of the aircraft but did not observe any damage. A subsequent inspection by an engineer revealed that the left forward wing attachment fitting had failed, and the left wing main spar was twisted.