This chapter explores teacher mental health as a fundamental yet often overlooked dimension of educational quality and sustainability. Grounded in the relational and emotional nature of teaching, it emphasizes that the heart of schooling lies not only in knowledge transmission but also in the meaningful human connections forged in classrooms. The chapter highlights the dual impact of teacher-student relationships, showing their potential to promote or hinder student development and teacher well-being. It advocates for educational policies that centre on the mental health of teachers, recognizing that ensuring their psychological support is essential to creating equitable, effective, and nurturing learning environments.