Ability-grouping pupils within schools, also referred to as attainment-grouping (Taylor et al., 2018) or tracking, has a long history in the United Kingdom and has attracted much research and debate (Ireson & Hallam, 1999). In primary schools in the United Kingdom, two main types of between-class ability-grouping are practiced: streaming and setting. Streamed pupils stay in a group of children with the similar ability for all lessons, while set pupils are placed in an ability group only for certain lessons (Ireson & Hallam, 2001). The term "tracking," which is often used in the U.S. literature, refers to practices analogous to setting or streaming (Gamoran & Nystrand, 1994). Within-class ability-grouping is a third type of ability-grouping and involves teachers organizing pupils into small groups by their skill levels. The three types of ability-grouping are not mutually exclusive, and children can be streamed, placed in sets, and allocated to within-class groups concurrently (Wilkinson et al., 2016).Within-school ability-grouping often starts early in the United Kingdom. Evidence from the Millennium Cohort Study (MCS), a U.K. population-based cohort of more than 19,000 children born around 2001, suggests that a significant proportion of U.K. primary school children, as young as seven, are in streamed classes (Hallam & Parsons, 2013b); at age 7, some 16% of the MCS children were streamed, 64% of whom were also set for literacy and 70% for mathematics. Nearly 26% of children were set for both literacy and maths, and 11% were set for only maths (8%) or literacy (3%). Within-class grouping appears to be the most prevalent practice with 79% of MCS pupils in England reported to be in-class grouped at age 7 (Campbell, 2014).