The age grouping of students, that is the homogeneous grouping of students based on the single factor of age, has been a standard feature of public schools since the establishment of massive public education during the American Industrial Revolution. Prior to this period the mixed-age, or multigrade grouping of students was the de facto standard in small rural schools. However, this was not so much theoretically or empirically derived but a practical issue because of a smaller numbers of students. While many educators have advocated for other approaches to the grouping of students, the state of evidence to date provides little consensus on what is most effective and age grouping remains the norm.