According to the flight instructor, during a training flight, he instructed the student pilot to perform a simulated power-off approach to a private turf airstrip. The student pilot was briefed to execute a go-around upon reaching 100 ft above the airstrip surface. About 100 ft above the airstrip, with the airplane configured with full flaps and the airspeed about 60 kts, the student was instructed to go-around. However, the airplane continued to descend, and the airspeed decreased to about 45 kts. The instructor applied full power and raised the flaps, but the airplane "touched down firmly" about mid-field off the left side of the runway and subsequently veered further left into a wheat field. The instructor visually inspected the airplane from his right seat and decided to takeoff to the south, returning to the home airport. A postlanding inspection revealed the left wing had sustained substantial damage. About the time of the accident, the local wind was from 180° at 28 kts and gusting to 35 kts.