According to the certified flight instructor (CFI), while demonstrating a grass strip departure to a student pilot, he stated that he taxied to a grass strip runway. He instructed the student pilot to conduct the takeoff. The student pilot advanced the throttle and according to the CFI; full takeoff power was achieved. Approximately half way down the runway the airspeed reached minimum rotation speed. The CFI stated that the runway began to upslope. The airspeed increased at a reduced rate, but the engine continued to operate normally. When the airplane reached the top of the slope, airspeed increased to 60 mph and the CFI called out “rotate”. The CFI assisted the student pilot with the rotation. The climbout was normal according to the CFI and the airplane struck a power line approximately 150 feet from the end of the runway 28. The airplane sustained substantial damage to the airframe. The CFI reported no preimpact mechanical malfunctions or failures with the airplane that would have precluded normal operation.