The Norwegian educational system is largely governed by egalitarian principles. It features a comprehensive public school system that is free of charge, employs a common curriculum, and does not use tracking or grade promotion/retention. Additionally, Norway provides universal healthcare, early education, and social support services. Despite these characteristics, recent evidence suggests a widening achievement gap between students whose parents have the highest and lowest levels of income and education (Sandsor et al., 2023). Furthermore, the proportion of children growing up in poverty has increased over the last decade. Internationally, research points to reduced access among lower-income students to supportive home environments (Davis-Kean et al., 2021), high-quality early childhood education (Cloney et al., 2016), and experienced teachers (Borman & Dowling, 2008). These factors are critical for long-term educational success (Dearing et al., 2024). However, most of these findings originate in more segregated societies and little is known regarding the mechanisms driven the SES achievement gap in the Norwegian context. Against this background, we analyzed population-level data to test how changes in family socio-economic status are associated with students’ academic achievement. Our study uses a random-intercept cross-lagged panel model to examine (1) whether within-student increases in family income are associated with changes in academic performance between 5th-, 8th-, and 9th-grade, and (2) the extent to which these relations vary across gender. By examining these questions within a welfare-rich and egalitarian society, this study contributes to understanding how longitudinal associations between family income fluctuations and children’s academic trajectories manifest in contexts with extensive social support systems and universal educational provision.

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