Cepeda, N. J., Pashler, H., Vul, E., Wixted, J. T., & Rohrer, D. (2006). Distributed practice in verbal recall tasks: A review and quantitative synthesis. Psychological Bulletin, 132(3), 354–380. https://doi.org/10.1037/0033-2909.132.3.354
This meta-analysis pools hundreds of experiments on the spacing effect and concludes that spreading study sessions over time produces better long-term recall than massed practice, with the ideal gap growing as the delay before the test increases. Its scale and its publication in a leading review journal make it one of the most authoritative sources on the topic, although most included studies used word lists rather than classroom material. It provides the core evidence for my argument that revision timetables should be spread across the term, and its lab-based limits point me toward classroom studies to complete the picture.