The student was undertaking his first instructional flight. At about 2000 feet during the climb towards the training area, the engine failed completely. A successful forced landing was carried out, however during the landing roll the gear struck a number of rocks. Initial investigation revealed a defect in the fuel cock selector mechanism, in that, irrespective of the cockpit indication, the fuel supply cock remained closed. The cable connecting the fuel shut off control in the cockpit to the fuel cock was deformed to the extent that the cock remained in the closed position. A note in the aircraft Flight Manual requires the cockpit control to be lockwired, as the manufacturer had evidently not intended that the control would be frequently manipulated. However, the instructor was not aware of the Flight Manual requirement, and had demonstrated the shut off control on numerous occasions over the past two years. He stated that he had not used the control before the flight in question, and enquiries did not reveal who might have operated the control in the period following the previous flight of the aircraft.