This paper addresses the appraisal of the quality of student debate about a socio-scientific issue, with a focus on the interactional factor. The “scientific café” pedagogical situation offers an opportunity for observing the circulation of arguments from small group to class-level debate, indicating the quality of group reasoning. We use Mercer and Wegerif’s typology of student talk (exploratory, cumulative, disputational) and study the attitude toward self-identity needed to engage in exploratory talk. Such a positioning is conceptualized as being associated to specific politeness rules and face-preservation system. Three case studies held in an American and a French high schools explore the boundaries between those 3 categories and questions which units of analysis are relevant for assessing students’ argumentation. The paper provides methodological tools to apply this typology to authentic students’ interactions, where students’ engagement in a specific group talk type is both a matter of cognitive skills and perceived relevancy in a given social context. We propose a refinement of the analytical categories that allows studying students’ argumentation at different time scales (interactional phase, sequence, subsequence) and social levels (group talk, individual self-identity footing, and repercussions on class debate).