DURING TAKEOFF FROM A SMALL LAKE, THE INSTRUCTOR PILOT ALLOWED A STUDENT PILOT TO MAKE A FLAT, STEP TURN TAKEOFF. DUE TO LOW FUEL IN THE TANKS, THE FUEL PICKUP UNPORTED AND THE ENGINE STOPPED AND STARTED SEVERAL TIMES DUE TO FUEL STARVATION. THE INSTRUCTOR WAS UNABLE TO RECOVER ENGINE POWER OR RETURN TO THE LAKE PRIOR TO HITTING TREES.