My high school radio shop had metal bench tops. At one time or another, most of us learned the reason for the “one hand in the pocket” rule.
Even this would boil down to, what could the teacher be reasonably expected to do. So what if the teacher was close, if his back was also turned and nobody was giggling and pointing or otherwise drawing attention to the soon to be almost Darwin award student. If the class requires a squad of private eyes to keep it safe from this kind of lawsuit, then litigation has gone way out of whack.
“Oh, I don’t know, it seems like he should have a little bit of liability for not recognizing the possibility that he’d get a student stupid enough to ignore the lecture on resistances in series and hook up a near short through his heart.”
That does sound like a pretty stupid thing to have done, even if not dangerous in and of itself, for just this reason. I mean, did he actually expect all of these kids he was teaching to understand a concept like that? Asking too much, I think.