This article examines the impact of a statutory assessment in England, the Phonics Screening Check (PSC), on classroom practices of grouping children by 'ability.' Bearing in mind the argument that assessment is the rudder that steers the otherwise slow-moving battleship of educational practice, it is argued that the PSC has altered how teachers organise their classes and curriculum in both the affected year group (Year 1, children aged 5-6) and in earlier and later years. Using data from a nationwide survey of teachers (n = 1,373), focus groups and in-depth interviews with teachers, the article examines how this relatively new phonics assessment forms part of a 'policy storm' of pressures relating to accountability, which encourage teachers to place children in groups on the basis of ability, even when they have doubts about this practice and there is little evidence to suggest grouping improves attainment. Practices include grouping children within classes, across year groups or even across several year groups, by phase of phonics learning, guided by advice from bought-in private phonics schemes. There is also evidence of 'educational triage,' where borderline children become the focus, and increased use of interventions which involve withdrawing children. Overall, the article uses the PSC to demonstrate how, in times of multiple policy pressures, assessment can rapidly alter practice, in this case making grouping a 'necessary evil', as one teacher respondent argued.