INSTRUCTOR AND STUDENT PLTS ABOARD C150 PERFORM FORCED LANDING DUE TO FUEL EXHAUSTION.

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I WAS CONDUCTING A 1 HR LESSON IN A C150 WITH A 10-FLT HR; PRE-SOLO STUDENT PLT ON A CLR CALM DAY. WHEN I ARRIVED AT THE ACFT THE STUDENT HAD COMPLETED THE PREFLT INSPECTION; RPTING 5 QUARTS OF OIL AND THAT THE FUEL IN EACH TANK CAME HALF-WAY UP HIS FINGER WHEN HE INSPECTED THE TANKS INDICATING 3+ HRS OF FUEL ON BOARD. I DID NOT CHK BEHIND THE STUDENT AND A FUEL DIPSTICK WAS NOT AVAILABLE. 0.9 HRS INTO THE FLT THE ENG'S RPM BEGAN TO INTERMITTENTLY INCREASE AND THEN DECREASE. I ASSUMED THE FLT CTLS AND SIGHTED A FIELD TO CONDUCT A FORCED LNDG. BECAUSE THE PWR DECREASE AND INCREASE WAS RHYTHMIC; I ATTEMPTED TO GAIN ALT AND CONTINUE TOWARDS THE ARPT THINKING THAT THE PROB WAS MECHANICAL IN NATURE. I CHKED THE POS OF THE KEY; PRIMER; MASTER SWITCH; CARB HEAT; MIXTURE; THROTTLE; AND FUEL SHUTOFF VALVE. AS THE PERIODS BTWN PWR LOSS AND GAIN LENGTHENED. I REALIZED THAT ATTEMPTS TO GAIN ALT OR PROGRESS TOWARDS THE ARPT WERE FUTILE. AT THIS POINT I NOTICE THAT THE L FUEL GAUGE INDICATED NEAR EMPTY AND THE R NEAR A QUARTER TANK. I ESTABLISHED A TFC PATTERN FOR THE SELECTED FIELD BASED ON THE FIELD LENGTH AND TREE HEIGHTS ON EACH END AND LANDED THE ACFT; APPLIED BRAKES; STEERED CLR OF OBSTACLES (HOUSE) AND CAME TO REST IN THE BRUSH AT THE END OF THE FIELD. NEITHER ME NOR MY STUDENT WAS INJURED AND THE ACFT HAD ONLY MINOR DAMAGE TO THE WING'S LEADING EDGE DUE TO A SMALL TREE IN THE BRUSH LINE. UPON IMMEDIATE INSPECTION OF BOTH TANKS I FOUND THE L TO BE EMPTY AND THE R TO BE VERY NEAR EMPTY. I CONTACTED THE ARPT TWR VIA CELL PHONE AND RPTED THE INCIDENT AND THE FAA WAS INFORMED. I CALLED THE CLUB'S CHIEF FLT INSTRUCTOR WHO CAME TO THE SCENE. WE WAITED FOR THE FAA TO ARRIVE. I COMPLETED A RPT WITH THE INVESTIGATOR ON THE SCENE. HE SAID THAT THE EVENT WOULD BE WRITTEN UP AS AN INCIDENT. I WILL NOW ALWAYS VISUALLY VERIFY FUEL REGARDLESS OF STUDENT LEVEL OF EXPERIENCE. AS A FLYING CLUB; WE NOW MANDATE THE ABOVE PRACTICE AND ALSO REQUIRE THAT ALL PLANES BE TOPPED OFF AFTER EVERY FLT AND THAT FUEL DIPSTICKS BE IN THE POSSESSION OF ALL PLTS BOTH STUDENT AND INSTRUCTOR. IN 120 HRS OF DUAL GIVEN; I HAD ONLY CONDUCTED 2 FLTS WITHOUT TOPPING OFF TANKS PRIOR -- THOSE 2 FLTS WERE IN C172 WITH A DIPSTICK PRESENT. I WILL NOT AGAIN DEVIATE FROM THIS PERSONAL PRACTICE.