The flight instructor reported that he was flying an instructional cross-country flight with a student. They stopped for fuel (elevation 250 feet) and were returning to their home base. When they departed, the wind was from 300 degrees at 16 knots, gusting to 20 knots. They decided to do a pinnacle landing to a 4,060-foot mountain. As the student approached the peak for landing, a downdraft was encountered. The main rotor disk low rpm indicators activated, and the flight instructor took the controls and attempted to salvage the situation. The helicopter's landing skid impacted terrain, and the aircraft rolled twice down the mountain. The flight instructor and student exited with minor injuries.