What was considered discipline back then(I can still taste the eraser dust and see the welts on my knuckles) would be considered assault today
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It was assault.
It was common in the public schools, too. In fourth grade in my public school, I saw a teacher break a girl’s wrist by shaking her and then slamming her across against the blackboard.
The gym teacher routinely disciplined the boys by making them bend over and then kicking their backsides.
Discipline was simply a lot harsher in all school settings until the last few decades.
It was assault.
Perhaps in some specific circumstances, but generally, not. Corporal punishment works.