PRIOR TO TAKEOFF ON A DARK NIGHT, THE DUAL STUDENT CHANGED THE FUEL SELECTOR FROM ONE MAIN TANK TO ANOTHER. AFTER TAKEOFF, AT ABOUT 150 FEET ABOVE GROUND LEVEL, THE ENGINE STOPPED DUE TO FUEL STARVATION. THE FLIGHT INSTRUCTOR STATED THAT THE FUEL SELECTOR WAS NOT IN THE DETENT, AND THAT HE HAD INADEQUATELY MONITORED THE STUDENT'S POSITIONING OF THE FUEL SELECTOR.