At 1500 central time, a Piper PA-18A-150 lost engine power during initial climb from Northwest Regional Airport and was damaged in the forced landing. The flight instructor and pilot receiving instruction suffered minor injuries. The pilot was receiving a flight checkout for his tail wheel endorsement and had been doing touch and goes. The pilot said that, when the flight departed a half hour earlier, the left fuel tank indicated approximately 3/4 full of fuel, and the right tank indicated approximately 1/2 full of fuel. During the initial takeoff climb of the 3rd or 4th touch and go, the engine lost power. The instructor switched tanks but the engine did not respond. The FAA inspector said the left tank was empty and the right main fuel tank contained plenty of fuel.