This chapter aims to bridge the gap between theory and practice in the fields of human development and education within the context of adolescence. It views the notion of “loneliness and mentalization within the classroom” from the dual lens of a developmentalist and a practitioner. Empirical evidence on social cognition and well-being will be critically discussed from a developmental approach, followed by a summary of practical educational suggestions. Specifically, this chapter addresses the paucity of research on adolescents’ perceptions of loneliness and feelings of isolation, and how this may be linked to Theory of Mind skills or the ability to understand the thoughts and emotions of self and other. This chapter engages the two disciplines of developmental cognitive psychology and learning sciences and may be useful for developmental interventionists who care to address the needs of the “whole child” in secondary school education. This chapter may thus appeal to a diverse array of educators and researchers who work with youth in ToM development and well-being. Socioemotional health and mentalization skills will be emphasized within a framework of a critical review of cutting-edge psychoeducational research.