The way in which school leaders implement professional learning communities (PLCs) is important for realizing sustainable school improvement. The assumption is that school leaders act based on their interpretation of the PLC, which is based on their underlying educational beliefs. In this study, we explored these latter aspects by interviewing six formal and informal school leaders, discussing the sustainability of PLCs in two secondary schools that had each worked with PLCs for seven years. The results of this in-depth qualitative study show that the schools differed in the degree of sustainability they achieved: one still used the PLC as intended; the other now omitted one step of the original format. This can be explained by the leaders' interpretations of the PLC and their educational beliefs. Furthermore, we found that student- and collaboration-oriented beliefs are critical for continuing the work of a PLC. Finally, we found that the organizational context mediates whether leaders act upon their beliefs. Tensions between leaders' beliefs and the organizational context, such as fear of colleagues' resistance, appeared to influence their choice to act or not concerning specific aspects of the PLC. These findings can give school leaders insights into conditions for sustainable school improvement with PLCs.