Teachers’ innovativeness and pedagogical knowledge are crucial to developing and implementing inno-vation sustainably in schools. Whereas schools often lack resources and capabilities to innovate, exter-nal knowledge sources can essentially enrich schools’ innovation processes. How external knowledge is valued, acquired, and used depends on individual attitudes, organisational capacities, and knowledge management. On this behalf, school leaders are considered crucial knowledge brokers and facilitators of change that can foster innovation. The impact of external knowledge sources on teacher innovative-ness, mediated by schools’ pedagogical absorptive capacity, school leaders’ Not Invented Here syn-drome, and knowledge-sharing practices, are presented. A random sample of German school leaders was analysed through structural equation modelling, mediation analysis, and latent moderated structur-al equation modelling. The results indicate that pedagogical knowledge absorptive capacity is neces-sary for schools to benefit from external knowledge sources. School leaders’ Not Invented Here syn-drome and knowledge-sharing are significant antecedents of collaborative innovation and teachers’ innovativeness. Fostering school leaders’ openness toward external knowledge, schools’ absorptive capacities, and developing strategic knowledge-sharing practices are vital to enhancing teacher innova-tiveness and the development of innovative teaching practices.