This article employs a mixed-method approach to assess the effects of El Salvadorian Integrated Systems of Full-Time Inclusive Schools (SI-EITP), which offers in-service teacher professional development (TPD) combined with a socioemotional learning intervention, on teacher well-being. Findings from the cluster-randomized controlled trial with no baseline show null results for most teacher well-being outcomes analyzed. Interviews suggest limited effects on teachers' mindfulness and emotion regulation and indicate that teachers' participation in TPD was potentially increasing their stress levels and emotion exhaustion. We recommend that TPD program designs consider how the delivery mode and intensity may influence teachers' stress.