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They also used to crank up their cars to turn them on. Today’s world is a different world. Today’s kids are not those of earlier generations. And today’s teachers are not those of earlier generations.
There are many mitzvos that are completely dependent on the outcome. You might check out Mesilas Yeshorim Perek 20, where he addresses the mitzvah of rebuke. Clearly, any form of tochacha that produces an opposite result is prohibited. In fact, he even states that it would be a mitzvah to not offer that rebuke!
I am not sure of the technicality of the rebbe’s potch. But if it produces the outcome that the kid straightens out, behaves, and does not lose cheishek for learning, it should fall within the “potch that contributes to chinuch”. If not, it is not permitted. The rebbe who harms a talmid is a problem, and should really be employed in another field. That’s why it is academically sound to be aware of what was done in the past, but that we must be cognizant of the facts on the ground when we want to apply old methods to a new phenomenon. Considering the risks involved, I would approach this cautiously.