A programme of resources and activities relating to ‘Social and Emotional Aspects of Learning’ (SEAL) has been rolled out nationally to primary and secondary schools in the UK, but we know little about how variations in the implementation of this work relate to key indicators of school success. In the present study, a team of experienced school advisors used a semi-structured observation and interview protocol to rate various aspects of the implementation of SEAL in 49 primary and secondary schools. A total of 2242 pupils in 29 of these schools completed measures of social experiences and school ethos. School-level attainment and attendance statistics were collated for all participating schools. Analysis revealed that ratings indicative of a whole-school universal approach to SEAL were significantly associated with school ethos, which in turn mediated associations with pupils’ social experiences, overall school attainment, and persistent absence. Thematic analysis of the advisors’ records illuminated key dimensions and exemplars of whole-school implementation. Results highlight the role of school ethos in systematically connecting whole-school practices relating to SEAL with key indicators of school success. Directions for further longitudinal work to elucidate specific causal mechanisms are discussed.