While learning outcomes in low- and middle-income countries are generally at low levels, the degree to which students and schools more broadly within education systems lag behind grade-level proficiency can vary significantly. A substantial portion of existing literature advocates for aligning curricula closer to the proficiency level of the "median child" within each system. Yet, amidst considerable between-school heterogeneity in learning outcomes, choosing a single instructional level for the entire system may still leave behind those students in schools far from this level. Hence, establishing system-wide curriculum expectations in the presence of significant between-school heterogeneity poses a significant challenge for policymakers -- especially as the issue of between-school heterogeneity has been relatively unexplored by researchers so far. This paper addresses the gap by leveraging a unique dataset on foundational literacy and numeracy outcomes, representative of six public educational systems encompassing over 900,000 enrolled children in South Asia and West Africa. With this dataset, we examine the current extent of between-school heterogeneity in learning outcomes, the potential predictors of this heterogeneity, and explore its potential implications for setting national curricula for different grade levels and subjects. Our findings reveal that between-school heterogeneity can indeed present both a severe pedagogical hindrance and challenges for policymakers, particularly in contexts with relatively higher levels of performance and in the higher grades. In response to meaningful between-system heterogeneity, we also demonstrate through simulation that a more nuanced, data-driven targeting of curricular expectations for different schools within a system could empower policymakers to effectively reach a broader spectrum of students through classroom instruction.