The student pilot was preparing for an instructional flight and, at the instructor's request, completed a preflight inspection of the airplane on the ramp while the instructor was inside completing dispatch paperwork. The student then entered the cockpit, where she prepared fro a cold start as it was the airplane’s first flight of the day. Prior to the engine start, she performed the engine start checklist; however, she did not set the parking brake, as she assumed it had been engaged when the airplane was parked the previous evening. Upon engine start, the airplane rolled forward. The student pilot attempted to stop the airplane but was unable to apply brake pressure because the seat was not properly adjusted, and she could not fully reach the brake pedals. Subsequently, the airplane collided with two unoccupied parked vehicles which resulted in substantial damage to both wings. Postaccident examination of the airplane by maintenance personnel revealed that the parking brake was not fully set. According to the flight school's chief instructor, the company's standard procedure is to keep the parking brake disengaged while the airplane is parked overnight. The flight school reported that it used the airplane manufacturer’s Pilot Operating Handbook (POH) checklist for preflight and engine-starting procedures. The POH “Before Starting Engine” checklist specifies adjusting the seat position as step three and setting the parking brake as step six. The chief instructor also stated that it had always been assumed that students would wait for their flight instructors before starting the airplane’s engine.