“If for instance a boy and a girl played truant they would both have been placed in detention but the boy would have been caned as well.” Girls only became liable to be caned following the arrival of a headmistress.
This might partly be due to the idea that “a gentleman never hits a lady” but that ignores the fact the misbehaving schoolgirls are not, by any stretch of the imagination, “ladies” and doesn’t prevent their being sent to a female teacher to be punished. Nor does it prevent a male teacher imposing some other punishment.
There is also the myth that girls are delicate creatures who need to be protected, by “big strong men”, from the realities of life.
Another factor is the sexist idea that girls are feeble minded and, as such, should not be held accountable for their actions. Again, men have an interest in perpetuating that myth whereas women have an interest in debunking it.