According to the flight instructor, the airplane landed without incident during the instructional flight. During the landing roll he intended to retract the flaps but instead placed the landing gear switch in the "up" position. The flight instructor immediately returned the landing gear switch to the "down" position; however, the landing gear collapsed. The airplane slid about halfway down the runway before it came to rest, which resulted in substantial damage to the fuselage. The flight instructor reported no preimpact mechanical malfunctions or failures with the airplane that would have precluded normal operation. According to the pilot operating handbook, "never rely on the safety switch to keep the gear down during taxi or on take-off, landing roll, or in a static position. Always make certain that the landing gear switch is in the down position during these operations."