Body mass index (BMI) is known to vary by individual characteristics, but little is known about whether BMI varies by school and by school characteristics.Nationally representative samples of United States schools and students are used to determine the extent to which BMI and percent of students at or above the 85th percentile of BMI vary by school and by school characteristics. Data from the 1991-2004 Monitoring the Future (MTF) study were analyzed in 2006 and 2007.A relatively small proportion of variance in BMI lies between schools; intraclass correlations are on the order of 3%. Still, this is sufficient variation to provide very different environments for students attending schools that are low versus high in average BMI. There is some modest variation by school type (public, Catholic private, non-Catholic private); school size (number of students in the sampled grade); region of the country; and population density. There is more variation as a function of school socioeconomic status (SES) and racial/ethnic composition of the school. School SES in particular was negatively associated with BMI levels, even after controlling individual-level SES and racial/ethnic status.The residual differences in BMI by school suggest that some characteristic of the school and/or community environment--perhaps cultural factors or peer role modeling or differences in school food, beverage, or physical education policies--facilitate obesity in schools with a high concentration of lower socioeconomic students, beyond individual-level factors.