Psychologists know how to read this data. It’s trivial with training in many cases. Sometimes very complicated in other cases, though.
But in terms of the question as to how teachers knew the cane worked with boys with little risk of harm – they knew it by using it and observing how the boys behaviour improved and how they reacted. And also, in many cases, by having been there themselves and they listened to each other too.
They also knew that it worked far less effectively with girls, and was more likely to do harm.
As above, how could they know that?
Exactly the same way.