This paper analyses how 8th graders build epistemic practices of evaluation of knowledge. Guided by Ethnography in Education, we followed a group in their science lessons through one year, using participant observation. Following Kelly's propositions concerning epistemic practices, we selected key events towards an analysis of discursive interactions. The results indicate a trajectory of changes in constructing practices of evaluation of knowledge over time. The repertoire built by the group throughout 8th grade allowed students to evaluate knowledge claims in order to value alleged scientific evidence to the detriment of other rationalities. Evidence, in this case, was being used as a tool for students to support and communicate their propositions, but they were not evaluated. Events that broke with rationality based on the use of evidence generated opportunities for the group to start engaging in practices such as: evaluating the quality of alleged scientific evidence, pondering possible errors, or proposing alternative interpretations. Future events in the history of the group throughout 9th grade indicate that this type of practice became part of their discussions. We discuss implications for pedagogical practice and research in science education.