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.
Research from the 1940s until recently that I am aware of. I’m not sure what you mean by ‘those policies’ though.
If they didn’t use it with girls, they couldn’t know how effective it was with girls nor whether it would do harm.
Plenty of teachers did use it with girls.
Most of the published research claims CP is harmful for both boys and girls.
Most of the credible, recent, research by people who actually know what they are doing and not just trying to achieve particular results does not.