Vygotsky's 'zone of proximal development (ZPD)' has become associated with the individual 'scaffolding' of learners. As a result, because teachers need to teach the whole class, many public school teachers have had to dismiss the concept as unworkable. Yet Vygotsky himself was chiefly concerned with public school teaching and firmly rejected the idea of a 'pedagogical duet' between learner and teacher. He also dismissed the teacher who attempted to provide the ensemble of learning content by him or herself as a 'rickshaw puller', arguing instead that a teacher should be a 'tram driver', who organizes the social environment of learning. One way in which the teacher might do this is by mediating a learning task for a single learner or a group of learners, who then mediate the task for their classmates in group-work. We present evidence that in this situation the way in which learners mediate tasks differs from the way in which teachers do, and argue that this suggests learner-to-learner mediation is in important ways closer to what Vygotsky termed 'internalization'. We believe that T-S and S-S interactional mediation do not create two different ZPDs but may instead lie within a single, whole class ZPD.