WHILE BEING TOWED DURING A TAKEOFF ROLL, THE DUAL GLIDER STUDENT WAS NOT ABLE TO MAINTAIN DIRECTIONAL CONTROL. THE INSTRUCTOR PILOT TOOK CONTROL OF THE AIRCRAFT, AND ELECTED TO CONTINUE THE TAKEOFF. HIS REMEDIAL ACTIONS WERE INADEQUATE TO KEEP THE AIRCRAFT FROM DEPARTING THE RUNWAY, AND HE DID NOT INITIATE AN ABORT UNTIL AFTER THE GLIDER STRUCK A TAXIWAY MARKER.