THE DUAL STUDENT (A PRIVATE PILOT WITH SINGLE ENGINE LAND RATING) WAS PRACTICING TOUCH AND GO LANDINGS AT DUSK, WITH A CERTIFICATED FLIGHT INSTRUCTOR IN THE RIGHT SEAT. DURING THE LAST LANDING, THE AIRCRAFT LANDED HARD. AN INSPECTION OF THE AIRCRAFT REVEALED THAT THE MAIN SPARS WERE BENT BEYOND REPAIR, FUEL WAS LEAKING FROM BOTH ENGINE GASCOLATORS, WING SKIN WAS WRINKLED, AND THE EMERGENCY LOCATOR TRANSMITTER WAS JARRED FROM ITS MOUNT AND HAD ACTIVATED.