Research has demonstrated that cognitive and motivational teacher characteristics are crucial for students' development. Surprisingly less is known about teachers' negative affective characteristics and students learning. Therefore, we investigated the association between teachers' emotional exhaustion and students' achievement in mathematics. Moreover, we explored whether this relationship is moderated by characteristics of class composition. The analyses are based on a representative sample of 1102 elementary school teachers in Germany and their students. Using multilevel analyses we found that teachers' emotional exhaustion is significantly negatively related to students' mathematics achievement. Moreover, results revealed a significant moderation effect, which indicates that the association between teachers' exhaustion and students' achievement is higher in classes with a high percentage of language minority students.