Background: An earlier study in a school in England found that a series of brief values affirmation writing exercises, performed over the course of a school year by students aged 11-14, increased the mathematics attainment of students of low socioeconomic status (SES). Aims: This pre-registered follow-up of the original study aims to investigate the long-term effects of values affirmation on low-SES students' attainment. Sample: The sample consisted of all students in the analytical sample of the original study who remained at the school and for whom the necessary data were available, N = 409 (95 low-SES). Methods: The students' results in high-stakes national standardized assessments at age 16, taken two to four years after the affirmation, were analysed. Results: The evidence did not support the pre-registered hypotheses that values affirmation would raise the attainment of low-SES students in mathematics and English. However, exploratory analyses suggested that for low-SES students in two of the three-year groups, the intervention increased Attainment 8, a broad policy-relevant measure of academic attainment, and increased the attainment of boys in English (in particular English Literature) but reduced the corresponding attainment of girls. Conclusions: The results suggest that the benefits of values affirmation can differ by student cohort and by school subject and that they might be time-limited in some circumstances. This suggests a set of hypotheses that future research could test in order to advance understanding of when values affirmation is, and is not, successful for school students over a sustained period.