This article argues for a conceptualization of school writing that emphasizes how cognitive and sociocultural factors interact. In academic, professional, and affinity-based communities, an emphasis on standards of quality drives the adoption of such practices as revision and peer and expert review. In school, everyday writing practices center around demonstrations of knowledge (taking notes, writing summaries, providing personal responses to readings, completing worksheets, etc.) where the teacher is often the sole or primary audience. This context tends to privilege knowledge-telling strategies and prioritize fluency and efficiency of expression. When these sociocultural factors interact with cognition, group differences tend to emerge, as knowledge telling provides few opportunities for disadvantaged groups to overcome barriers linked to differences in prior knowledge, fluency, attention management, or motivation. It also affects the efficacy of instructional practices, because cognitive ability and social identity affect how students internalize the metacognitive concepts and strategies characteristic of expert writers.