The certificated flight instructor and his student were on approach, in a glider. The student was attempting to make a no-airbrake landing. While on approach, the student was using a forward slip to correct the altitude. The instructor reported that his student’s aggressive control inputs made the aircraft too slow and too low to reach the airfield. The instructor took over the controls and decided to make an off field landing. While landing, the glider’s right wing impacted tall grass. The flight instructor reported that the aircraft ground looped to the right causing substantial damage to the rudder and empennage. Neither the flight instructor nor the student reported any mechanical failures or malfunctions of the airplane.