INSTRUCTIONAL FLT ENDS WITH AN OFF ARPT LNDG BECAUSE OF FUEL STARVATION. THERE WAS FUEL ONBOARD.

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A STUDENT AND MYSELF WERE ON AN INST TRAINING APCH AT THE MDA 1/2 MI N OF RWY WHEN THE ENG BEGAN RUNNING ROUGH. AT THAT POINT I HAD HER SWITCH TANKS AND AT 450 FT AGL REALIZED I COULD NOT MAKE THE RWY WITHOUT CAUSING MAJOR INJURY AND DAMAGE ON A CIRCLE TO LAND APCH. I THEN LANDED STRAIGHT AHEAD IN A PLOWED FIELD OFF ARPT. THERE WAS NO INJURY OR DAMAGE AND THE ACFT WAS FLYING WITHIN A FEW HRS. PRIOR TO THE FLT I ASKED THE 100+ HR PLT HOW MUCH FUEL WAS IN EACH TANK AND SHE REPLIED 'UP TO THE TABS' WHICH IS MINIMUM 17 GALS/TANK IN A PA28161. DURING OUR APCH CHKLIST WE TURNED THE FUEL PUMP ON AND PUSHED THE MIXTURE TO FULL RICH POS AND I TOLD HER NOT TO SWITCH TANKS BECAUSE AFTER A 1.2 HR FLT WE HAD MORE THAN 1 HR OF GAS LEFT. IT SO HAPPENED THAT WE LEFT WITH ONLY 8 GALS IN THE FUEL TANK AND SHE MISREAD THE FUEL LEVEL. AFTER SWITCHING TANKS; THERE WAS NOT ENOUGH TIME FOR THE FUEL LEVEL TO FILL THE LINE AND RESTART THE ENG. THIS COULD HAVE BEEN AVOIDED BY MYSELF PAYING MORE ATTN TO FUEL MGMNT.