Well, if you want anecdotal, almost every family I know where there are boys and girls, the girls just seem to find school work easier than the boys.
My Mum has a sister and a brother, the girls sailed through A-levels and went to uni, the boy had to retake O-levels.
My BIL was educated privately because he failed the 11+ whereas his sister went to grammar school.
A good friend has a DD who is expected to pass 11+ this time, her DS struggles in middle-lower sets at comp
DH, left school, to join the Army at 16, his sister went on to A-levels and beyond.
I don't think it's because the boys are less bright, but that school doesn't suit them in the same way. When they start school, schools are almost universally run by women, sitting still and listening just doesn't suit most young boys and by the time they get to secondary, they've been taught that they're not "good" at school.
I think maybe girls are more disposed (conditioned?) towards wanting to conform and impress their parents/teachers and doing as they're told.