"he should have been fired over it and never been able to get another job as a welding teacher."
Even hypothetically, that seems pretty harsh for a situation about which you know none of the particulars. Having been associated with the welding program at the local community college for about ten years now, I can tell you with absolute certainty that there will never be a shortage of high school and college age students who will ignore any safety rules, find ways to defeat any precautions, and generally manage to damage equipment you would not even believe they could damage. To prevent any possibility of accidents you would have to have one attendant per student, and probably one attendant per attendant. Even if that absurd situation were economically possible, it would only reduce the number of accidents, not prevent them entirely.
Every incident has to be evaluated and judged on its own circumstances.