I might just be able to help with this one, I saw an almost identical incident a few years ago in the lowveld, in which a 172 xp had the same symptoms, excepting that the Instructor and Student were able to return to the airfield after a quick but interesting circuit, on landing they tried a run up and found nothing wrong, so rather than take a risk, they called the flight school to say AOG, come fix. The following day we went to collect the A/C, we swapped the fuel divider on the engine and ran it it, all seeemed OK so we did a quick but very interesting test flight around the circuit, problem was still there.
After scratching heads and checking the engine, we had a look again into the cockpit to see if anything had been missed, what we found was a small unmarked knob similar to a vent air actuator, to the right of the mag switch, it was pulled about one third out from the dash, as this was the only thing sticking out, we pushed it in and tried again, hey presto, problem fixed, turns out to be a fuel cut off switch..
can't rememeber if it was a standard fitting, but the aircraft in question was ZS-JXA