The certified flight instructor (CFI) and a student pilot were practicing an engine failure, with a 180-degree turn, in a Schweizer 269C helicopter. According to the CFI, the student pilot attempted to "stretch," to reach a field to the helicopter's left, even though a more obtainable field was located to the helicopter's right. The CFI called for a go-around at 150 feet above the ground; however, when the student pilot rolled on the throttle and pulled up on the collective, the rpm began to drop quickly, and the helicopter continued to descend. The CFI took the controls; however, the helicopter struck pine trees, and descended to the ground. Examination of the helicopter by a Federal Aviation Administration inspector did not reveal any preimpact malfunctions; nor did the CFI report any. The CFI reported 340 hours of total flight experience, which included 238 hours accumulated in helicopters.