Mashing a students head with a metal dumbbell is a bridge too far, but its a reflection of the sorry state that our schools are in, and a reflection of the current breach between adults (in authority or otherwise) and children. Once, the fact that a grown person COULD batter you quite easily was reason enough to not provoke them too far, whoever they were. There has been a terrible reverse where now a generation of adults fears its children.
At least one paper reported that students had visited this man's house and threatened him on previous occasions. It appears this went a little deeper than cheeky kids winding someone up.
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"There is a time when the operation of the machine becomes so odious, makes you so sick at heart,
that you can't take part" [Mario Savo, 1964]