I was doing some research for a paper and noticed that an enormous body of research points out that grouping students by ability does not result in any improvement in learning. This is especially true for the low group, whose members actually read far less and make fewer gains in ability-grouped classrooms. There's some evidence to suggest that the high group suffers too. Despite this, the majority of elementary classes in our district have the three reading groups. Some teachers group across the entire grade level. Why does this persist?