The status of the phoneme /s/ as the only sibilant of Finnish makes its pronunciation relatively free. This enables /s/ variants to gain social meaning, a tendency typical in many societies. In Finnish society, studies so far have documented how variation in /s/ pronunciation has faced concerns, originating from late-nineteenth-century nation building and Finnish language norm construction processes. Against the norm of the voiceless alveolar /s/, fronted variants first represented Swedish influence and a threat to norms of ‘good Finnish’, later meeting more global indexes. The historical development of the /s/ ideology is still echoed in the contemporary social meaning potentials of /s/ variation. By focusing on learning materials used in the Finnish education system during the period from the 1900s to the 1970s, this article investigates how formal education has contributed to the ideology of the (im)proper Finnish /s/, manifested in the ideological construct of ‘Helsinki s’.