Over the last 20 years, the use of administrative data has become central to understanding pupil attainment and school performance. Of most importance has been its use to robustly demonstrate the impact of socio-economic status (SES) on pupil attainment. Much of this analysis in England and Wales has relied on whether pupils are eligible for free school meals (eFSM). However, very little is known about the validity of this measure as a proxy for SES. Using a recent major birth cohort study, this paper examines the relationship between pupils' eFSM and their more detailed socio-economic circumstances.