Background There is growing research support for the use of mindfulness training (MT) in schools, but almost no high-quality evidence about different training models for people wishing to teach mindfulness in this setting. Effective dissemination of MT relies on the development of scalable training routes. Objective To compare 4 training routes for school teachers wishing to deliver MT differing in intensity and potential scalability, considering teaching competency, training acceptability, and cost-effectiveness. Methods Schools were randomized to an existing route comprising an 8-session instructor-led personal mindfulness course, combined with 4-day MT program training, or 1 of 3 more scalable, lower intensity, alternatives: an instructor-led personal mindfulness course combined with 1-day MT program training, a self-taught personal mindfulness course (delivered through a course book) combined with 4-day MT program training, and a self-taught personal mindfulness course combined with 1-day MT program training. Results Attrition from training was substantial across all routes. The instructor-led course was more effective than the self-taught course in increasing teachers’ personal mindfulness skills. Even the most intensive (existing) training route brought only 29% of the teachers commencing training, and 56% of those completing the study protocol, to the required minimum competency threshold (an advanced beginner rating on an adapted version of the Mindfulness-based Interventions Teaching Assessment Criteria). The differences in levels of competency achieved by existing training compared with the more scalable alternatives were modest, with economic evaluation suggesting that the existing route was both more expensive and more effective than lower intensity alternatives, but with no statistically significant differences between routes. Conclusions This research questions the move toward abbreviating teacher training to increase scalability and suggests instead that many teachers require additional support to ensure competency from first delivery of MT in the classroom.