Whereas growth in international school numbers is widely reported, less attention has been given to how these schools have developed as organisations. Drawing on organisational life-cycle models (Greiner, 1972) and the work of DiMaggio and Powell (1983), this paper addresses that gap. As international schools grow individually, and as the field expands collectively, processes of institutionalism are affecting the legitimacy of claims to 'international' status and, this paper argues, are also normalising organisational shape, structure, form and function. Schools (and their leaders) face isomorphic pressures to mimic each other, are being coerced into similar form and are adopting field-wide normative practices. The paper concludes, however, by showing that culturalist perspectives and institutional entrepreneurship offer an alternative. Reproduction of organisational form may to some extent be inevitable, but that reproduction is moderated by diversity and can be manipulated and resisted by school leaders strong enough to escape the iron cage.