When were the studies done? Those policies were in place (in some schools) in the 1960’s so, unless the research was done significantly before then, those policies couldn’t be based on it.
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.
If they didn’t use it with girls, they couldn’t know how effective it was with girls nor whether it would do harm.