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, no 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 the 1950s, there was a trend to appoint a female deputy if the head was male. In other cases, a Senior Mistress was appointed. Their job being mainly confined to disciplining of the girls.