The proportion of adolescents in the United States who are obese has nearly tripled over the last two decades. At the same time, schools, often citing financial pressures, have given students greater access to "junk" foods and soda pop, using proceeds from these sales to fund school programs. We examine whether schools under financial pressure are more likely to adopt potentially unhealthful food policies. Next, we examine whether students' Body Mass Index (BMI) is higher in counties where a greater proportion of schools are predicted to allow these food policies. Because the financial pressure variables that predict school food policies are unlikely to affect BMI directly, this two step estimation strategy addresses the potential endogeneity of school food policies. We find that a 10 percentage point increase in the proportion of schools in a county that allow students access to junk food leads to about a one percent increase in students' BMI, on average. However, this average effect is entirely driven by adolescents who have an overweight parent, for whom the effect of such food policies is much larger (2.2%). This suggests that those adolescents who have a genetic or family susceptibility to obesity are most affected by the school food environment. A rough calculation suggests that the increase in availability of junk foods in schools can account for about one-fifth of the increase in average BMI among adolescents over the last decade. (Data tables are appended. Contains 28 footnotes and 7 tables.)