"They estimated that there were about 30 gallons of fuel on board prior to departure; 17 gallons in the left tank and 13 gallons in the right tank. After airwork and emergency procedures practice, they returned to the departure airport for some takeoffs and landings. During the second approach to landing, about two hours into the flight, the student pilot advanced the throttle; however, the engine did not respond."
If they had done one check correctly they would have made the initial takeoff with the left tank selected since it was fullest. It contained an estimated 17 gals. A fuel burn of 8.5 gallons (US) per Hobbs hour is about right for a PA-28-180. NTSB found left tank selected at crash site with about half gallon remaining.
That suggests to me that the right tank was never selected during the two hour flight.