Part of the problem is terminology confusion, too. I used to get around this by calling the mags OFF check (rotary switch) the NO LIVE MAG check, and the Left-Right check the NO DEAD CUT check. With throttle closed (500-600 RPM or so) the same check (BOTH-R-L-OFF-BOTH) was conducted after startup and before shutdown, this helped to reinforce the difference between these two (really the same) and the MAG DROP check, which of course, early students tended to do - and at 1700 RPM or whatever the mags were turned to OFF. As an instructor, I always anticipated this possibility with early students and had my left hand near the mixture control, and watched the student's hand like a hawk - you could tell by the hesitant way they operated the key that the old OFF job was on the cards. Turning the key to OFF will not damage anything - apart from making the next start more difficult with an empty carburettor -but turning it back ON again might, of course. So quickly pulling the mixture to ICO was the answer, to prevent the student turning the key back to BOTH. Of course it meant a bit of embarrassment at the holding point, but obviously better than the possible alternative.
DH