Re: "...it happens a lot earier than college"
Unproven and doubtful.
People keep pretending we're living right after the 1960s. It is now 2015. I would say my US middle school classes were evenly balanced between the men and the women. And yes, I was in the highest academic group at my school. We were all taught algebra at the same time and we all progressed through geometry, trig, and calculus. As these were essentially the required track for academics, the numbers in those classes didn't change. I will observe however that for the only optional math class I took (probability and statistics), the numbers fell off by about half.
Now when you look at the sciences it is a whole other story. There we had some flexibility. While everyone took something they called Earth Sciences (I can't even recall exactly what they taught in that one) and Chemistry, in the following years we had choices of Biology, Anatomy, Physics, Nuclear Science, Geology and Astronomy. A fair number of the women ditched Physics in favor of Anatomy and there were none in the Nuclear Science group. Sadly it turned out the Geology and Astronomy classes were the ___ for Jocks classes.
When I got to college, there was one woman in my Astronomy group and maybe three in our Physics (boffins not engineers) group.
Whatever choices they are making, it is NOT societal pressure which contributes to them. The only ones pressuring anyone are the ideologues who are committed to the idea that the science must equally represent women even if the women are freely choosing otherwise.