...the share of opposite gender friends has a sizable negative effect on high school GPA. The effect is found across all subjects for students over the age of sixteen, but is limited to mathematics and science for younger students.
From a research paper (PDF) by Andrew Hill at the University of South Carolina.
My mother was always wary of me chasing after girls in high school because she was worried it would interfere with my studies. Perhaps she was on to something.
(h/t Marginal Revolution)