WHEN ENTERING THE DOWNWIND LEG THE PVT PLT DUAL STUDENT REPORTEDLY SWITCHED THE FUEL SELECTOR VALVE OF THE FULLEST TANK. THE ENGINE CONTINUED TO OPERATE NORMALLY UNTIL THE LOCAL CONTROLLER INSTRUCTED THE FLIGHT TO GO AROUND, WHEN THE ENGINE SUSTAINED A TOTAL LOSS OF POWER. THE CFI ASSUMED COMMAND OF THE AIRPLANE AND EXECUTED AN EMERGENCY LANDING. THE AIRPLANE STRUCK A DIKE AND NOSED-OVER INTO A SLOUGH. THE CFI STATED THAT HE SUSPECTED THAT THE FUEL SELECTOR VALVE WAS NOT PROPERLY SEATED IN THE DETENT POSITION.