Different studies suggest that collaborative argumentation among peers promotes school learning, especially the comprehension of concepts. However, the available evidence shows that the relationship between argumentation and learning is not direct but instead mediated by development processes that, in turn, promote learning. The goal of this study is to understand the mediating role that the development of argumentative inner speech may play in the process of constructing knowledge through collaborative argumentation. A case study was conducted in which one child (fourth grade) was tracked throughout an entire unit in which he and his peers argued collaboratively class after class. We assessed the students individually before and after in their learning (oral and written) and written argumentation skills. The collaborative work from all the classes was videoed and analysed through discourse analysis. The student showed significant progress in both delayed learning and written argumentation compared to the group. Furthermore, the analysis of oral tests shows that the argumentative interactions that initially appeared in the discussions among peers were internalized so he could understand the concepts involved on an individual level. The article discusses the implications of these results in understanding the role of discursive interactions in school learning processes.