Teacher innovativeness and pedagogical knowledge are crucial for sustainable development and implementation of innovation initiatives in schools. Although schools often lack resources and capabilities to innovate, external knowledge sources can essentially enrich their innovation processes. How external knowledge is valued, acquired and used depends on individual attitudes, organisational absorptive capacities and knowledge management. Therefore, school leaders are considered crucial knowledge brokers and facilitators who foster innovation. This study explores the impact of external knowledge sources on collective teacher innovativeness, mediated by schools’ pedagogical absorptive capacity, and school leaders’ knowledge-sharing practices and not-invented-here syndrome. A random sample of German school leaders was analysed through structural equation modelling, mediation analysis and latent moderated structural equation modelling. The results indicate that pedagogical knowledge absorptive capacity is vital for schools to ensure that they benefit from external knowledge sources. School leaders’ not-invented-here syndrome and effective knowledge-sharing are significant antecedents of collective teacher innovativeness. Fostering school leaders’ openness towards external knowledge, schools’ absorptive capacities and developing strategic knowledge-sharing practices are essential for enhancing teacher innovativeness and developing innovative teaching practices.