I've said that I think that elite schools are holding their enrollment of female students to 50%, even though women are going to college at higher rates and are generally outperforming men in high school. As evidence, I present the following chart:
The black line is the percentage of female students enrolled in 4-year college degree granting institions by year, according to the Digest of Education Statistics. You can see that this percentage is about 55% today.
The orange line is the percentage of females offered admission to Princeton by year, according to the Daily Princetonian (which generally reports these numbers in April of each year). You can see that this percentage has been about 50% since 2000. The enrollment figures crossed the 50% line around 1988.
These numbers don't tell the whole story. A particularly important mitigating factor for Princeton is that the number of men and women applying to Princeton each year is about equal. I'm not sure why this is—it certainly seems to be outside of Princeton's direct control. In any case, given the generally better qualifications of women, I think it would serve Princeton well to recruit and admit more of them.