Background: Little is known about adolescents' food purchasing behaviors in rural areas. This study examined whether purchasing food at stores/restaurants around schools was related to adolescents' participation in school breakfast programs and overall diet in rural Minnesota. Methods: Breakfast-skippers enrolled in a group-randomized intervention in 2014 to 2015 (N = 404 from 8 schools) completed 24-hour dietary recalls and pre/post surveys assessing food establishment purchase frequency. Healthy Eating Index Scores (HEI-2010) were calculated for each student. Student-level school breakfast participation (SBP) was obtained from school food service records. Mixed-effects regression models estimated: (1) whether SBP was associated with store/restaurant use at baseline, (2) whether an increase in SBP was associated with a decrease in store/restaurant use, and (3) whether stores/restaurant use was associated with HEI-2010 scores at baseline. Results: Students with increased SBP were more likely to decrease fast-food restaurant purchases on the way home from school (OR 1.017, 95% CI 1.005, 1.029), but were less likely to decrease purchases at food stores for breakfast (OR 0.979, 95% CI 0.959, 0.999). Food establishment use was associated with lower HEI-2010 dairy component scores (p = 0.017). Conclusions: Increasing participation in school breakfast may result in modest changes in purchases at food establishments.