School performance is a critical aspect of adolescents’ lives. Several factors have an impact on school performance. The aim of this study is to analyze the relevant personal and contextual variables associated with top performance and low achievement in a sample of Portuguese and Spanish adolescent students. The sample included 1,564 adolescents, mean age 14 years old, and was collected from the HBSC (Health Behavior in School-aged Children) survey. The questions in this study covered sociodemographic, health and wellbeing, health-related behaviors, family, school and peers. Results show that students with low performance more frequently have worse social-contextual and personal/health-related indicators, while the opposite is the case for top performers. Student-teacher relationships appeared as the most influential variable on school performance paths, χ2(2) = 328.11, p < .001; but other variables within families, e.g. mother studies, χ2(2) = 50.54, p < .001, and schools, e.g. liking the school, χ2(1) = 16.27, p < .001 and χ2(1) = 22.54, p < .01 (in the low and high student-teacher relationship branches of the decision tree, respectively), as well as some health and wellbeing variables, e.g. health related-quality of life, χ2(2) = 53.58, p < .001, and χ2(2) = 63.86, < .001 (in the low and high student-teacher relationship branches, respectively), appeared significant in the paths.