RPTR FLT INSTRUCTOR ALLOWS SPI TO PULL MIXTURE CTL INADVERTENTLY INSTEAD OF THROTTLE CAUSING OFF ARPT FORCED LNDG.

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THIS WAS AN INSTRUCTIONAL FLT IN VFR CONDITIONS; LCL TOUCH AND GO'S FLT IN A CESSNA 150. WE TOOK OFF AT XA15 EST. COMPLETED THE TOUCH AND GO'S; GOT CLRNC TO LAND ON OUR DOWN WIND LEG. WENT THROUGH NORMAL TFC PATTERN PROCS. ON ABOUT A 1/4 MI FINAL I TOLD MY STUDENT TO REDUCE PWR BECAUSE WE WERE GETTING HIGH ON THE VASI GS. INSTEAD OF PULLING THE THROTTLE BACK; HE PULLED THE MIXTURE FULL LEAN. A FEW SECONDS LATER; I POINTED OUT TO HIM THAT WE WERE STARTING TO GET LOW ON THE GS. HE TRIED TO GIVE PWR BUT TO NO EFFECT. WHEN I LOOKED TO HIS HANDS; I SAW THE MIXTURE FULL LEAN. HE HAD STARVED THE ENG OF FUEL. I IMMEDIATELY PUSHED THE MIXTURE IN AND TRIED TO RESTART THE ENG. IT WOULDN'T CATCH. BY THAT TIME WE WERE TOO LOW AND SLOW. I DECLARED AN EMER TO THE TWR AND TRIED TO GLIDE THE PLANE TO THE RWY. WE WERE TOO LOW AT THAT POINT. SO; I PUT THE PLANE ON THE GRASS JUST BEFORE THE RWY. THE GND WAS TOO SOFT. AS SOON AS THE NOSE GEAR CAME DOWN; IT STARTED TO SINK IN THE SOFT GND AND COLLAPSED. THE PLANE WENT NOSE OVER AND FLIPPED. MY STUDENT AND I SUFFERED NO INJURIES AT ALL.