The certificated flight instructor, seated in the right front seat, was conducting a Title 14, CFR Part 91, instructional flight with a student pilot seated in the left front seat, who had accrued 4 flight hours. A second student occupied a rear seat to observe the training. The instructor reported that she planned to have the student fly over the runway in a landing configuration. The student pilot established the airplane in a slow flight maneuver at 65 knots with the flaps down, while flying about 5 to 10 feet above the runway. The instructor reported that the airspeed dropped quickly from 65 to 45 knots, and the airplane began to descend. She instructed the student to "go around," but when the descent rate was not arrested, she took the controls. At that time, the airplane was in a nose-low, right bank. The instructor ensured that full power had been applied, raised the nose of the airplane, and applied left rudder and left aileron. The airplane continued to descend, and the left wing struck the runway. The instructor regained control of the airplane after touchdown, and taxied to parking. The airplane received structural damage to the left wing. The instructor reported that the airplane did not have any preimpact mechanical malfunction.