According to the certificated flight instructor, he and the student pilot/owner had been performing takeoffs and landings for about one hour with the left fuel tank selected when they switched the fuel selector valve to the right fuel tank and performed a "runup." During the takeoff climb, approximately 300 feet above ground level, the engine experienced a total loss of engine power. The flight instructor assumed control of the airplane from the student pilot/owner and attempted to land in a clearing. During the landing flare the right wing impacted a tree resulting in substantial damage to the wing. A Federal Aviation Administration inspector that responded to the accident location examined the airplane and the engine. He noted that the left fuel tank was full of fuel, the right fuel tank, carburetor, and electric fuel pump were devoid of fuel and no other preimpact anomalies were noted.