In this 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 predicted improved peer relationships, better overall school attainment, and reduced 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..