It's actually a global phenomenon. Boys are performing worse in most of the Western world as well as much of the rest. When there are similar issues in the US, Japan, and Scandinavia you can rule out many cultural aspects as the cause.
Which is hardly surprising.
For example in 2010 in the Netherlands (from http://barrolee.com/) 45% of women aged 30-34 have some tertiary education and 31% have completed tertiary education (31 / 45 = 69%) whereas the male statistics are 39% and 25% (25 / 39 = 64%). This is then offset by women working fewer hours per week (from Eurostat): in the Netherlands men average only 35 hours per week, and women only 25. Dutch working hours are a little extreme, but the general trend is worldwide.