In my experience, girls were, on average, better students than boys. Thus the higher grades. The few really brilliant math students, however, were mostly boys.
I think one of the bad things for boys was combining them with girls in classrooms. Both sexes develop in different areas at different times - a girl is generally ready to handle a pencil, for instance, sooner than a boy is.
This causes problems for both, because there will naturally be periods in development when the boy excels, and vice versa.
I think we should go back to all girl and all boy classrooms.