Increasing dropout rates in senior high school physical education, particularly among females, and unhealthy activity and obesity levels in youth have led to recommendations to assess potential contributing factors in physical education participation. Drawing from gender, body image, and social-cognitive theory, this study investigated relations between body size discrepancy, self-efficacy, test anxiety, and achievement in 316 high school physical education students. Gender differences were noted in body size discrepancy (females reported the desire to have a smaller body). Specifically in females, body size discrepancy predicted test anxiety, which predicted self-efficacy. Self-efficacy predicted achievement in both males and females. The results signal that gender-specific relations among these constructs are important factors to consider in the achievement scores of students in high school physical education. Physical education programs should model curricula and instructional practices that defuse potentially harmful body image discrepancies that seem most poignant in females while engaging all learners to feel competent and safe. (Contains 2 figures and 2 tables.)