We conducted three preregistered studies using the Organization for Economic Co-operation and Development Programme for International Student Assessment (PISA) data to provide a worldwide estimation of the standardized test gap between students from lower and higher social classes. We investigated: (a) the degree to which academic anxiety contributes to this gap and (b) the role of country-level income inequality in widening this gap. In Study 1, we used PISA 2003 data (250,000+ students from 41 countries) and demonstrated that anxiety accounts for approximately one-fifth of the performance gap between students with less educated parents and those with more educated parents. Unexpectedly, the social class test gap was weaker in more unequal countries than in more equal countries. In Studies 2a and 2b, we used the PISA 2012 and 2015 data (totaling over a million students from 65 countries and 72 countries, respectively) and differentiated the cultural dimension (parental education, cultural capital) and the economic dimension (economic capital) of social class. Regardless of the dimension, anxiety again accounted for between one-tenth and one-fifth of the performance gap between students from lower and higher social classes. Moreover, (a) the culturally based social class achievement gap was weaker in more unequal than in more equal countries, and (b) the economically based social class achievement gap was larger in more unequal than in more equal countries. Unexpectedly, we also find a robust association between national income inequality and academic anxiety across all three studies. Results are discussed in relation to the multidimensionality of social class and literature on the psychology of income inequality.