Research on stereotype threat, which is defined as the risk of confirming a negative stereotypic expectation about one’s group, has demonstrated that the applicability of negative stereotypes disrupts the performance of stigmatized social groups. While it has been shown that a reduction of stereotype threat leads to improved performance by members of stigmatized groups, there is a lack of clear-cut findings about the mediating processes. The aim of the present study is to provide a better understanding of the mechanisms that stereotype threat causes in women working on mathematical problems. In addition, the study set out to test stereotype threat theory in a natural environment: high school classrooms. The experiment involved the manipulation of the gender fairness of a math test. The results indicate that the stereotype threat effect exists in this everyday setting. Moreover, it appears that dejection emotions mediate the effect of threat manipulation.