This is the third publication in a series of research reports from the UCL Centre for Educational Leadership-led study, The Impact of the ECF Programme on the Work Engagement, Well-Being and Retention of Teachers: A Longitudinal Study, 2021--2026. This paper describes the conditions in schools which advance the development and retention of teacher mentors on the Early Career Framework (ECF) programme. A significant part of our analysis was aimed at better understanding the "paths" of influence connecting school leadership to mentors' retention decisions through Job Satisfaction and Well-Being. School leaders retain high quality teachers by helping to create productive work environments aimed at achieving ambitious, socially valued goals for all students. Given similar student populations, a high-retention school is doing many things well, a lowretention school, not so much. This paper concludes with five recommendations designed to inform the work of school leaders wishing to retain their current ECF mentors and teachers and to respond to the much broader challenge of school improvement. Each of these recommendations will add significantly to the school's "stickiness" for teachers and should contribute to student success, in the long run. [For Report #2, see ED631331.]