Background: Through their leadership, principals have an important, yet typically indirect, effect on pupils' learning. This article focuses attention on principals' influences on teaching and assessment in special education in Sweden: specifically, in special schools for pupils with intellectual disabilities (SID). Whilst SID principals are accountable for teaching and assessment quality in their schools, research on this aspect is limited. Purpose: The research sought to investigate principals' leadership practices within SID in terms of their influence on teaching and assessment. Methods: Data in this qualitative study comprised six principals' written reports, produced as part of assignment work during a principal education programme. These related to their systematic examination of the internal processes at their schools in relation to teaching and assessment. The reports were analysed, in depth, using an approach informed by the theory of practice architecture. Findings: The analysis suggested that principals' influences on teaching and assessment were constrained, and enabled, by a number of factors. Constraints included arrangements which shaped a form of leadership characterised by replicating the organisational structures of mainstream schools. However, enabling arrangements, including the increased distribution of time and space, emerged as principals' perspectives shifted. It may be possible for principals to increase their influence on teaching and assessment in SID by better understanding their practice architecture in relation to other practices. This could influence the practice ecology in relation to teaching and assessment, and increase the improvement capacity of the local school - and beyond - in terms of teaching and assessment. Conclusion: This study draws attention to SID principals' self-understanding as system players, and the opportunities to influence practice ecology. It highlights how principals' leadership within special education has the potential to improve teaching and assessment in local schools within a community, ultimately becoming the driving force for realising inclusive education.