Higher academic performance is almost universally considered a good thing, and most quantitative studies show that performance is positively, although weakly, related to mental health. Simultaneously, however, qualitative studies consistently find that high-performing students and students attending high-performing schools report high levels of stress and other mental health problems. This study investigates a simple explanation for this puzzle -- that the relationship between performance and mental health is not linear and is conditional on the performance culture of the school. Data on almost 5000 Swedish students from the Programme for International Student Assessment were used. The results show that the relationship between performance and mental health is generally not linear and that intermediate-performing boys have the best mental health, while both low- and high-performing girls and boys alike have poorer mental health. Although inconclusive, the results also suggest that low-performing students may be vulnerable to a strong school performance culture.