During the instructional flight in the Schweizer SGS 2-33 glider a normal traffic pattern and normal approach were performed to runway 28. About 20 feet above the runway, the CFI instructed the private pilot to reduce the spoilers slightly and pull back on the stick. The private pilot then increased the spoiler application and moved the control stick forward. The glider immediately hit the runway nose low, bounced once and slid to a stop down a slope, beside the runway. No mechanical failures or malfunctions were reported with the glider.