Commonsense told teachers that corporal punishment was ineffective with the vast majority of girls. They could see that by observation and to how often behaviour improved or deteriorated after it was used. Commonsense may have also told teachers it was doing some harm, because they could see that in short term and medium term reactions.
It required research to be done to quantify that harm, and work out more precisely what form it was taking.
The primary reason girls schools used corporal punishment much less than boys schools was because they found it ineffective in comparison to other methods. They didn’t feel they needed it. Concern it might actually be harmful was much less important in that decision.