At 13:00 MST, a Piper PA-32R was damaged during a forced landing on a highway after the engine quit. The flight instructor and student were not injured. According to the flight instructor, the student was under the hood preparing for an approach to Casa Grande Municipal Airport. While attempting to switch the fuel selector to the fullest tank the student inadvertently selected the off position. When the engine quit, he read the emergency engine power loss checklist to the instructor, who performed the required actions. The student omitted the first item, Fuel selector-switch to tank containing fuel because he believed that he had already accomplished that item. The instructor was unable to restart the engine within the time and altitude remaining.