Neighborhood structural factors are associated with greater feasibility of youth active travel and thus, greater levels of physical activity. However, limited prior work has addressed walkability factors specific to the school neighborhood related to adolescent physical activity during the school day. Therefore, the purpose of this study was to examine the relationship between two school neighborhood walkability factors (neighborhood density and neighborhood age) and school-related adolescent moderate-to-vigorous physical activity (MVPA). We analyzed cross-sectional data from the Family Life, Activity, Sun, Health, and Eating (FLASHE) study for 1,183 adolescents ages 12-17 years. Separate linear regression models assessed the association between both school-neighborhood density and school-neighborhood age (i.e., population/housing unit density and age of buildings/units, respectively, within a 400m buffer around school address) and adolescent school-related MVPA, adjusting for age, sex, race/ethnicity, weight status, urban/rural environment and parent MVPA. School-neighborhood density (8.62, 95% CI: 0.70, 16.53) and school-neighborhood age (6.38, 95% CI: -0.29, 13.05) were both positively associated with school-related MVPA, although the neighborhood age-MVPA association observed was not statistically significant. When seeking to improve physical activity among adolescents, school neighborhood structural factors that support physical activity throughout a school day should be considered. Unfortunately, macro-scale environmental features such as these are not easily modified. Thus, these findings should be used to inform additional research related to school-neighborhood walkability and adolescent participation in physical activity.