Background Teachers' self-reported stress is related to the quality of teacher–student interactions and students' learning outcomes. However, it is unclear if teachers' physiological stress is related to child-centred teaching practices in the classroom and whether teaching practices mediate the link between teachers' stress and students' learning outcomes.

Aims We studied the effect of teachers' physiological stress and self-reported stress on their teaching practices and thereby on students' learning outcomes in math.

Sample A total of 53 classroom teachers and 866 Grade 1 students participated in the study.

Methods Salivary cortisol in the middle of the school day and cortisol slope from morning peak to evening were used as indicators of teachers' physiological stress, in addition to self-reported teaching-related stress. Teaching practices were observed with the ECCOM instrument. Students' math skills controlled for gender and previous skills were used as a measure of learning outcomes. Data were analysed with a two-level SEM.

Results Teachers' physiological stress did not have an effect on teaching practices or students' math skills. Teachers reporting less stress used relatively more child-centred teaching practices compared with teacher-directed ones. These practices had a marginal effect on classroom-level differences in the gain of students' math skills in Grade 1. There was neither a direct nor indirect effect from teachers' stress on students' math skills. Altogether, our model explained 77% of classroom-level variance in math skills.

Conclusions Teachers' self-reported stress has an effect on their teaching practices, which, in turn, have a marginal effect on students' learning outcomes.