In-country migration is widespread in South Asia, and the region hosts the world's largest number of out-of-school children. Yet the relationship between internal migration and inclusion in formal education has received only limited academic and policy attention. The Agenda 2030 pledge to leave no-one behind prompts us to argue that when it comes to migrating children, formal education systems in South Asia are in ontological crisis. Cases of mobile pastoralists and seasonal labour migrants illustrate that being 'left behind' is produced by intersecting norms of modernity, which formal education systems reproduce, via logics of rights, human capital and 'terms of inclusion'. Formal education systems are antithetical to mobility-dependent livelihoods, discount situated learning, and perpetuate unequal social relations. 'Alternative' education has done little to contest these norms and their productions. Terming this an 'ontological crisis' signals both the disjuncture between realities and Agenda 2030's moral exigencies, and the opportunity to act.