While on final during a practice instrument approach, the engine lost total power. An attempted engine restart and switching of the fuel tanks was unsuccessful. The flight instructor took control of the airplane from the pilot receiving instruction, and executed a forced landing to a corn field. The airplane sustained substantial damage to both wings. The airplane was recovered from the field and about 1 cup of fuel was found in the right fuel tank, and the left wing fuel tank was about 3/4's full. The engine was functionally tested for about 10 minutes on the airframe with no anomalies noted. The pilot reported he did not check the wing fuel tanks during pre-flight and relied on the cockpit fuel gauges for fuel quantity.