The failure of the dual student to maintain airspeed following total loss of engine power resulting in an inadvertent stall and subsequent uncontrolled descent. Also, inadequate supervision by the CFI for her failure to note the position of the fuel selector valve and her failure to maintain control of the airplane. Factors in the accident were the flight crews lack of experience in the accident airplane, improper painting of the arrow on the fuel selector handle by an unknown person, and the improper placement of the fuel selector handle to the off position by an unknown pilot resulting in fuel starvation.