The flight instructor and student pilot of the glider were under aerotow. The flight instructor reported that very early during the takeoff, the rear canopy opened. Sitting in the rear seat and controlling the glider, the flight instructor could not close the canopy in flight and believed that continuing the tow was a higher risk than releasing and landing straight ahead. When the glider was near treetop level, the flight instructor released the tow and elected to land in a harvested corn field directly off the end of the runway. Upon touchdown, corn stalks built up in front of the glider’s main wheel, bringing the glider to an abrupt stop upright and pivoting it 180º from the direction of landing, and resulting in substantial damage to the aft fuselage. The flight instructor reported that it was likely that he did not properly close and lock the rear canopy, and that the glider had no mechanical malfunctions or failures that would have precluded normal operation.