Grouping children by ability for reading instruction is common practice in many elementary schools today. By reducing heterogeneity, ability grouping presumably allows teachers to provide instruction at an appropriate level for students in a particular group. However, research has shown that grouping children by ability can have a negative impact on lower ability readers, especially when the grouping occurs over time.
Logan, N. L., Rux, J. D., & Paradis, E. E. (1991). Profile of a Heterogeneous Grouping Plan for Reading. Reading Horizons: A Journal of Literacy and Language Arts, 32 (2). Retrieved from https://scholarworks.wmich.edu/reading_horizons/vol32/iss2/1