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 acquire the Bokmål variety simultaneously through extracurricular exposure and thus develop what may be termed "bidialectal literacy". In this study, we correlate, 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. We suggest that this may be seen as an effect of the "bilingual advantage" in cognitive development and that such advantage may arise even in the case of closely related linguistic varieties.