According to the student pilot, while on the downwind leg to runway 26, he "knew that he was too high" for a normal approach. He deployed the spoilers to full and extended the downwind leg to lose altitude. While turning onto base leg for final approach, he retracted the spoilers to one half thinking that he was still too high in the turn. The pilot stated that he flew into a pocket of air that caused the glider to sink. The pilot realized that he was going to be "quite" low and stated that he retracted the spoilers all the way in. While turning on to final approach, he clipped a tree with the left wing and the glider spun around to the left and came to rest in a tree, 25 feet above the ground. Post-accident examination of the airplane by a Federal Aviation Administration inspector revealed that the left and right wing leading edges incurred substantial damage due to impact with trees. Both spoilers were found in the full extended position. The student pilot reported no pre-impact mechanical malfunctions or failures with the airplane that would have precluded normal operation.