The flight instructor reported that, after landing, he gave the controls to the student to let him taxi to the ramp. After entering the ramp, the student taxied the airplane towards an unoccupied parking spot. The instructor added that the student was taught to stop the airplane perpendicular to the landing spot, shut down the engine, and use the tow bar to back the airplane into the unoccupied spot, per company policy. The instructor further reported that as the student passed a parked fuel truck adjacent the parking spot, he maneuvered the airplane into the parking spot instead of following company policy and during the left turn, the instructor saw that the left-wing tip was getting close to the truck, so he applied brakes and told the student to stop the airplane, but the airplane's left wing struck the truck. The student reported that he did not follow company policy. The airplane sustained substantial damage to the left-wing spar. The chief flight instructor reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation.