Despite the striking reversal of the gender gap in education, women pursue science, technology, engineering, and mathematics (STEM) degrees at much lower rates than those of their male peers. This study extends existing explanations for these gender differences and examines the role of the high school context for plans to major in STEM fields. Building on recent gender theories, we argue that widely shared and hegemonic gender beliefs manifest differently across schools so that the gender-specific formation of study plans is shaped by the local environment of high schools. Using the National Education Longitudinal Study, we first show large variations between high schools in the ability to attract students to STEM fields conditional on a large set of pre-high school measures. Schools that are successful in attracting students to these fields reduce the gender gap by 25 percent or more. As a first step toward understanding what matters about schools, we then estimate the effect of two concrete high school characteristics on plans to major in STEM fields in college--a high school's curriculum in STEM and gender segregation of extracurricular activities. These factors have a substantial effect on the gender gap in plans to major in STEM: a finding that is reaffirmed in a number of sensitivity analyses. Our focus on the high school context opens concrete avenues for policy intervention and is of central theoretical importance to understand the gender gap in orientations toward STEM fields.