Alternatively, the 'boy problem' was always with us. The difference is post-school opportunities (and greater opportunity for women). 

During my school years in the 60s and 70s, girls did very well academically but then relatively closed doors to most professions sent them to nursing, teacher, bank teller or retail. Between the time my big sister graduated and when I graduated, the opening of opportunity for young women was evident as more and more went to university and professions.

Less than stellar performance by boys was quite evident when I was in school. But no one worried because one could exit high school into a decent paying low skill job. Higher competition and fewer opportunities is arguably exposing what was always there.

Important to remember that in absolute terms, more young men are entering university than ever before. It is only relative to young women there is the appearance of 'decline'.