The pilot stated that he completed the takeoff checklist and began the takeoff roll. The airplane accelerated, and at 75 to 80 miles-per-hour (mph) indicated, he attempted to rotate, but was unsuccessful because of what he thought was his seat restricting aft movement of the control stick. The certified flight instructor (CFI) attempted to move his control stick and was also unable. He (CFI) reduced the throttle and mixture controls, and both applied brake pressure. The airplane went off the end of the runway onto grass and came to rest with the nose landing gear collapsed. The CFI stated that, before takeoff, he confirmed freedom of the flight controls. During the takeoff roll when the airplane had reached 65 mph and the pilot had not begun to rotate he informed the pilot to “...lighten the nose....” The pilot acknowledged, and when at 85 mph, the CFI told the pilot to rotate. The pilot replied that he was trying. The CFI reported he attempted to rotate, applying aft elevator control, and he then felt metal on metal contact. Realizing that a control problem of unknown origin existed, he reduced the throttle and mixture controls, took control of the airplane, and asked the pilot to assist him on the brakes. He avoided the runway end lights and had the canopy released before contacting bushes at the edge of a lake. Both exited the airplane, and the pilot mentioned to him at that time that his seat had slid forward while turning onto the runway resulting in elevator control interference. Postaccident inspection of the airplane by a Federal Aviation Administration airworthiness inspector revealed the left seat was in the most forward position and the flight control stick was in contact with the seat. While in that position the elevator primary and secondary flight control surfaces were in positions corresponding to airplane nose-down input. The left seat was moved aft, and full elevator control authority was obtained. Further examination of the seat revealed the locking mechanism appeared to be operational; however, it was noted that the seat did not contain a cutout which allowed full aft movement of the control stick.