Because virtually every informed and experienced teacher was aware of the simple reality that the cane worked with a great many boys and hardly ever did them any harm if used appropriately.
How could they know it “hardly ever did any harm”? Boys were more likely to be subject to CP; in adult life, men are more likely to suffer depression and commit suicide. The obvious (although possibly incorrect) conclusion is the CP is harmful to boys.
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?