In American schools violence is often institutionalized. Teacher beat children, children get used to violence. The children are often not too reflected. People advocating violence as a tool for education are usually not big fans explanations and calls to reason. So children have no clue weather they are subject to education or sadism. Probably they are often at least partly subject to sadism.
And if they get used to the feeling that they are subject to the teachers sadism, it's no wonder that they see this as usual. Their educators and there for role models do, so why shouldn't they do?
Children probably do not think about what causes serious harm and what does not. Weather it was justified or not. Expically because it's only natural to assume that teachers punishment against oneself is usually not justified.
Tl;dr: if you beat children, to be surprised that said children beat other children.
/edit: disregard this, it's bullshit:
>Most urban public school systems, even in states where it is permitted, have abolished corporal punishment
https://en.wikipedia.org/wiki/School_corporal_punishment_in_...
Has this been permitted within the last 50 years? It definitely happened in the 1960s and earlier, but corporal punishment has not been a thing in American schools for a long time.
Corporal punishment is widespread in Alabama, Arkansas and Mississippi schools.
Corporal punishment is routine in some schools in smaller districts of Georgia, Louisiana, Missouri, Oklahoma, Tennessee and Texas.
It's only in 2011 that parents had the right to prevent their children being hit at school.
(This is all public schools. Hitting children in private schools is legal in all but 2 states.)
This isn't just children. Some 18 and 19 year olds attend schools and a subjected to physical violence as punishment.
Over 100,000 students received corporal punishment in 2011/2012. (And some of them will have been hit on more than one occasion)
I'm sure I'm not the only one who has noticed that many bullies stopped bullying you or even became friendly with you after you made a real effort to fight back.