The instructor and student pilot were practicing takeoffs and landings. On the second landing, when the airplane touched down, it veered sharply to the right. The instructor took control of the airplane and tried to stop the turn by applying full left rudder. She said the rudder peddle was too stiff and the airplane departed the side of the runway. The airplane struck a dirt berm and damaged the firewall and empennage. Examination of the airplane revealed there was a lot of debris and loose onjects in the airplane, but there was no physical evidence that the rudder pedals had been jammed or impeded. A functional test of the rudder pedals, brakes, and landing gear fork revealed no mechanical anomalies.