THE DUAL STUDENT WAS RECEIVING INSTRUCTION FOR HIS THIRD FLIGHT IN A TAIL WHEEL AIRPLANE. DURING THE TAKEOFF ROLL, THE AIRPLANE BEGAN TO DRIFT OFF THE RUNWAY. THE CERTIFIED FLIGHT INSTRUCTOR (CFI) ATTEMPTED TO TAKE REMEDIAL ACTION BY USING THE RUDDER PEDALS, BUT THEY WERE INOPERATIVE. THE AIRPLANE CONTINUED TO DRIFT OFF THE RUNWAY AND ONTO ROUGH TERRAIN. THE LEFT MAIN GEAR COLLAPSED. AN EXAMINATION OF THE AIRPLANE REVEALED THAT THE RUDDER PEDALS FOR THE RIGHT SEAT HAD BEEN STOWED, AND THIS WAS NOT DETECTED BY THE CFI PRIOR TO THE TAKEOFF ROLL.