Design-Based Learning (DBL) is a pedagogical approach where learning is situated in the context of design activities that are typically carried out in groups. During collaborative design inquiry, reflection is of vital importance for learning from design experience. The paper presents a research-through-design study with the intention to generate knowledge on how to design for supporting reflection-in-action during design-based learning. This study addresses the research question: how can a media tool support a group of students in externalizing reflection while they are engaging in DBL? This paper describes the design and evaluation of ReflectionSchema, a tool intended to support reflection-in-action in such a learning environment. The tool offers visualized guidance for coordinating a group of students' understanding of how they can combine reflecting with making design representations. The tool was trialed by 13 groups of students aged 12 to 13, who used the tool to record short reflection videos along their whole DBL process. We analyzed the efficacy of ReflectionSchema in terms of supporting collaborative reflection-in-action. Furthermore, we investigated the process of reflection-in-action during Design-based learning processes by examining the collaborative reflection patterns in the reflection videos that include the actions, roles, recording preference/style, and strategies employed. We discussed how these findings can inform the design of other reflection tools for similar settings.