On July 17, 2003, at 0941 mountain daylight time, a Cessna 172P, N98969, was substantially damaged when it struck two parked Cessna 172s, N54955 and N5484K, during takeoff at Front Range Airport (FTG), Watkins, Colorado. The certified flight instructor and her student were not injured. Visual meteorological conditions prevailed. No flight plan had been filed for the local instructional flight being conducted under the provisions of Title 14 CFR Part 91. The flight originated at Centennial Airport (APA), Englewood, Colorado, at 0730. According to the flight instructor after performing "four taxi back takeoffs" at APA the flight proceeded to FTG. On the second touch-and-go landing on runway 26, the flight instructor said she added full throttle and as the airplane started to rotate, her seat released and slid aft, causing her to lose her grip on the flight controls. The airplane departed the runway to the left, struck the ground, and traveled 785 feet across the grass between the A taxiway and the ramp before colliding with the two unoccupied airplanes, located on the ramp. The accident airplane's right wing was crushed down at the wing root and the wing strut was buckled. The outboard edge of the flap and inboard section of the right aileron was wrinkled. N54955's empennage was destroyed. The left wing tip was crushed and the left aileron separated on the outboard edge and was bent up. The left wing flap was crushed. N5484K sustained minor damage to the rudder. Post examination of the seat rails revealed no anomalies. When brought to the full forward position, the inboard seat pin would not engage fully into the seat track. When force was applied, the seat would move aft. According to a Cessna representative, the outboard seat pin was installed on the outboard side of the seat handle and should be installed on the inboard side of the seat handle. An examination of the remaining aircraft systems revealed no anomalies. The flight instructor was asked to explain how she adjusts her seat in the airplane. She replied that on that particular flight, she lifted the seat leaver, moved the seat full forward, released the leaver and rocked the seat to ensure that it was locked in the seat rail. The Cessna Model 172 Series Service Manual provides only an illustration addressing the installation of the seat pins. There is no textual data available for installation.