One also has to remember that the school leaving age was not always 16. It was 14 and later raised to 15, so this problem did not arise.
Also, looking certainly at girls today, not in a sexual way, but a 14-year-old girl today is far more physically mature than say in 1950.
that is just my view.
In secondary schools, certainly, after 1950, there was a trend to appoint a female deputy if the head was male.
In other cases, a Senior Mistress was appointed. Their job is mainly confined to the discipline of the girls.