Past research has indicated a significant relationship between physical fitness and standardized test scores; however, the relationship between physical fitness and other aspects of school performance has yet to be empirically examined in a population specifically composed of middle school girls. Because girls have a harder time transitioning through the middle school years, they are an important group to study in this context. This study examined several factors that contribute to school success, such as classroom behavior, attendance, and grades, in relation to physical fitness among a group of adolescent girls. It was specifically designed to examine the statistical relationship between physical fitness, as measured by the FitnessGram, and quantitative data on school performance including grades, standardized test scores, school behavior, and attendance among 280 middle school girls. The biopsychosocial theory was used as the basis of this study, with the biological factors of fitness levels and body mass index, psychological factors of grades and test scores, and social factors of attendance and behavior. A one-way between-subjects multivariate analysis of variance demonstrated that the psychological and social factors of school performance were significantly related to the biological factor of physical fitness. A statistically significant correlation was also found between body mass index, grades, and attendance. Interventions to increase physical fitness may be a way to foster greater school performance in middle school girls.