THE STUDENT GLIDER PILOT REALIZED THAT HE HAD DRIFTED TOO FAR FROM HIS DEPARTURE FIELD. HE ASKED THE INSTRUCTOR TO TAKE CONTROL OF THE AIRCRAFT. THE INSTRUCTOR CONSIDERED LANDING IN AN OPEN FIELD BUT DECIDED TO ATTEMPT TO RETURN TO THE AIRPORT. AS THE INSTRUCTOR MANEUVERED FOR THE APPROACH OVER A HIGH POINT OF GROUND, THE GLIDER STRUCK THE TOP OF A TREE AND CRASHED SHORT OF THE AIRPORT.