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Parents are concerned about the in influence of friends during adolescence. Using the gender composition of schoolmates in an individual's close neighborhood as an instrument for the gender composition of an individual's self-reported friendship network, this paper finds that 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. Self-reported difficulties getting along with the teacher and paying attention in class are important mechanisms through which the effect operates.