When the student pilot realized his altitude was high on final approach he lowered the nose and deployed the spoilers, which increased the rate of descent and airspeed in excess of what was required. After the flight instructor had the student close the spoilers, the glider encountered turbulence and a sink rate that put them at a low approach altitude, about 150 feet above ground level. The flight instructor then took control and performed an off-airport landing. During the touchdown the glider impacted a large shrub bush with its right wing, slid sideways through a second bush, and then completed a 270-degree turn before coming to rest upright. The impact with the shrub resulted in substantial damage to the leading edge of the right wing. The flight instructor reported no preimpact mechanical malfunctions or failures with the glider that would have precluded normal operation.