According to the student glider pilot, as well as his CFI who was observing from the ground, his base turn from the downwind leg was made at too low an altitude and too late, relative to his position in the landing pattern, to allow for arrival at the runway with sufficient altitude and airspeed for a safe landing. Contributing to the low and slow condition on final approach was the pilot's failure to realize the spoilers were still deployed. The glider collided with an airport perimeter fence short of the runway causing substantial damage.