This data sets contain data that were collected for a randomized evaluation in Kenya. In 2005, 140 primary schools in western Kenya received funds to hire an extra grade one teacher. Of these schools, 121 had a single first-grade class and split their first-grade class into two sections, with one section taught by the new teacher. In 60 randomly selected schools, students were assigned to sections based on prior achievement. In the remaining 61 schools, students were randomly assigned to one of the two sections. The ultimate purpose of the study is to assess the impact of tracking students into separate classes on student test scores and teacher effort.