Research in undergraduate physics and in K-12 science education has demonstrated challenges and successes in facilitating student engagement with reasoning practices associated with professional physicists. Here we focus on one important dimension of physics reasoning, using evidence to revise models. While this topic has been explored at the undergraduate level, less is known about younger students' physics reasoning, especially within the context of modeling and model revision, where measurement tools are less emphasized and where evidence is often observational qualitative data. Here we examine 7th graders' conversations about pieces of observational scientific evidence in their conceptual models of magnetism. We present a series of cases from a whole class discussion to illustrate students' productive reasoning using evidence while revising, challenging, and testing their scientific models. These cases illustrate how students reason about evidence in the context of canonical and noncanonical models of magnetism, specifically when different pieces of evidence contradict each other and when running the model is inconclusive. The results capture a variety of complex ways evidence can be productively used in reasoning with models, and, more importantly, show that middle school students' reasoning shares important similarities with more advanced physics reasoning, which can be leveraged when designing and building future instruction.