Prior research has demonstrated that teachers' professional knowledge and motivation are strongly related to students' learning and motivation. Symptoms of teachers' stress and burnout (e.g., emotional exhaustion) are also thought to influence students' achievement, but no empirical study has tested this prediction. Using multilevel analyses and a representative sample consisting of 1,102 German elementary school teachers and their students, we addressed this gap in knowledge by examining the association between teachers' emotional exhaustion and students' achievement in mathematics, and by testing whether classroom composition moderates this relation. We controlled for teachers' gender, their years of experience, their teaching certificate, and the composition of the class, and on the student level for students' gender, language spoken at home, socioeconomic status, and cognitive ability. Results revealed that teachers' emotional exhaustion was significantly negatively related to students' mathematics achievement, even after teacher characteristics and classroom composition were controlled for. Classroom composition moderated this relation, whereby teachers' emotional exhaustion was more strongly related to students' achievement in classes with a high percentage of language minority students. These results highlight the importance of teachers' well-being for students' learning.