School-based mindfulness programs have gained widespread interest as a means to support the development of social-emotional learning skills that contribute to student academic performance and well-being. As these programs are increasingly implemented by educators with varying levels of training and prior exposure to mindfulness, there is a growing need to evaluate teacher-led mindfulness instruction delivered in typical classroom or out-of-school settings. The TMEOS is a 28-item semi-structured observational scale designed to evaluate program fidelity and implementation quality. The observational protocol, which can be used by a mindfulness facilitator to rate new or practicing teachers, examines procedural adherence and embodiment of attitudinal foundations of mindfulness during program implementation across four domains. The domains include: (1) Planning, Organization, and Curriculum Coverage; (2) Teaching Mindfulness; (3) Guiding Mindfulness Practices; and (4) Management of the Learning Environment. Findings from initial validation studies have demonstrated that, with adequate training and preparation, acceptable levels of inter-rater and internal consistency can be achieved. The TMEOS is a promising observational scale appropriate for use in evaluating multiple dimensions of implementation quality of mindfulness instruction delivered in educational settings.