The commercial pilot receiving instruction reported that during the seventh practice autorotation to landing that day, the helicopter touched down on the aft part of the landing skids and started to roll forward. The pilot stated he applied aft cyclic to prevent the helicopter from rolling forward and one of the main rotor blades subsequently struck the tail boom. The flight instructor stated that he was unable to react in time to level the helicopter prior to touchdown. Examination of the helicopter revealed structural damage to the tail boom and tail rotor drive shaft. No mechanical anomalies were reported with the flight control system. The pilot stated in the "how could this accident/incident have been prevented" section of the Pilot/Operator Aircraft Accident Report form, "proper execution of a full down autorotation by fully leveling the aircraft before touchdown."