The private pilot said his approach to land was unstabilized. When the airplane was about halfway down the 3,000 ft-long runway and about 150 ft above the ground, the flight instructor took control of the airplane, added full power and attempted to go-around. The instructor said he retracted 10 degrees of flaps once he thought the airplane established a positive rate of climb; however, the stall warning horn sounded. He lowered the nose of the airplane to gain airspeed, but the stall warning horn continued to sound. To avoid powerlines and trees located off the end of the runway, the flight instructor turned to execute a forced landing on a road. He fully retracted the flaps, and the airplane clipped the power lines and impacted the road. The private pilot, who was an aviation maintenance technician, said the engine was operating normally and that the airplane just did not have enough airspeed and altitude to clear the power lines.