...one thing I did with all my primary students in the C150/152 is distract them enough I could flip the fuel shutoff without being seen.
Hardly a valid failure scenario - "Your passenger has just turned off the fuel"?
People might have failed to turn on the fuel prior to start and subsequently come to an unexpected stop during taxying, but unless you're flying something with a fuel selector which goes through 'OFF' when switching between 'LEFT' and 'RIGHT', I cannot see that deliberately turning off the fuel supply is a likely event.