The certified flight instructor (CFI) and student pilot planned to conduct an instructional flight in the tandem-seat, tailwheel-equipped airplane. The student pilot had a total flight experience of 20 hours, all of which was in the accident airplane make and model. He had not soloed yet. The reported wind was calm with 10 miles visibility. The student pilot was seated in the front seat of the tandem-seat airplane, and began the takeoff roll, his first for that day. The takeoff roll was described by the CFI as "a little wiggly," but was in accordance with the CFI's experience and expectations for a student pilot in a tailwheel airplane. Just after liftoff, the airplane drifted off the right side of the 75-foot-wide runway. The CFI did not take over the controls, since he did not perceive any immediate hazards. He then felt the airplane "hit something." He took over the controls, landed the airplane on the remaining runway, and the airplane was taxied to a parking spot. Postaccident examination of the airplane revealed that the fabric on the aft bottom fuselage was torn, and some of the fuselage structural tubing members were bent or broken. Additional examination revealed that an airport taxiway sign was damaged, and that debris from the sign was scattered on the runway. Witness marks indicated that the airplane had struck the top of that sign. The CFI reported that he never saw the sign during the veeroff, due to his rear-seat location. There were no reports by either pilot of a preimpact mechanical malfunction or failure with the airplane that would have precluded normal operation.