Based on Gross's process model of emotion regulation, this study related 53 lower-secondary school students' perceptions of their teachers' emotions to four of their teachers' emotion regulation while teaching. A mixed method approach, combining students' surveys and teachers' interviews, revealed associations between teachers' positive or negative emotions as perceived by their students, and teachers' reflections on their emotion regulation. Antecedent-focused emotion regulation appeared more desirable than response-focused emotion regulation, and in particular, reappraisal more effective than suppression in increasing positive-emotion expression and reducing negative-emotion expression. Implications for teaching, teacher education and future research on teacher emotion regulation are proposed.