The Norwegian language has two written standards, Bokmål (majority variety) and Nynorsk (minority variety), and children receive their schooling in one or other of them. Pupils schooled in Nynorsk learn the Bokmål variety concurrently through extracurricular exposure and thus evolve what may be labelled bidialectal literacy. This inquiry correlates, at municipal level, the results from Norwegian standardized national tests in reading, arithmetic, and English from four cohorts of eighth graders (2009–2012), with available statistics on language of instruction and socio-economic status. The finding is that municipalities with Nynorsk pupils have better than average results in national tests once socio-economic factors are taken into consideration. It is proposed that this may be seen as an effect of the ‘bilingual advantage’ in cognitive development and that such advantage may emerge even in the case of tightly related linguistic varieties.