DURING LANDING THE STUDENT PILOT DEVELOPED A LEFT DRIFT WHICH EVOLVED INTO A GROUND LOOP. THE TAILWHEEL IMPACTED A DRAINAGE DITCH AND MUDDY TERRAIN OFF THE LEFT SIDE OF THE RUNWAY. ON PREVIOUS FLIGHTS, THE INSTRUCTOR HAD FLOWN FROM THE LEFT SEAT BECAUSE THE AIRPLANE DID NOT HAVE BRAKE PEDALS INSTALLED FOR THE RIGHT SEAT. DUE TO DEMONSTRATED COMPETENCY BY THE STUDENT, THE CFI HAD ELECTED TO OCCUPY THE RIGHT SEAT DURING THE ACCIDENT FLIGHT.