Differential academic language proficiency is an issue of major educational concern, bearing on problems varying from pupil performance, to social prospects, and citizenship. In this paper we develop a conception of the language-acquiring subject, and we discuss the consequences for understanding differential language proficiency in schools. Starting from Wittgenstein's meaning-as-use theory we show that learning a language requires an activity that relates the subject both to the community of language users, and to the things language is about. In opposition to Luntley, we contend that this does not mean that linguistic development involves linguistic adjustment to the world "as it is". It is argued that, in as far as linguistic development involves a process of adjustment, this concerns conceptions about the world as it is presupposed to be--a "world" that is subjected to doubt and revision time and again. With respect to dealing with differential academic language proficiency, this approach to linguistic development suggests bringing pupils into situations which require active participation in processes of "negotiating meaning", including negotiating the prevailing presuppositions about what the world is like. This also puts novices in a different position--less assimilatory--recognising their co-constructive potencies at a more fundamental level.