Work-related burnout and stress-related sickness absence have become increasingly prevalent, but evidence on which workplace features shape workers’ mental health remains limited. Using population-level Swedish register data covering all lower- and upper-secondary teachers from 2006–2024, we show that schools serving more disadvantaged students exhibit substantially higher rates of sickness absence, particularly for stress-related diagnoses. Exploiting within-teacher variation across student cohorts, we separate sorting from exposure and find that a one standard deviation increase in student disadvantage raises overall and stress-related sick leave by 3.6% and 8.7%, respectively. Survey evidence indicates that these effects operate through classroom conditions rather than workload or organizational differences. The findings establish client composition as a distinct and policy-relevant determinant of worker health in contact-intensive occupations.