Background: Systematic barriers have hindered girls’ (across all ethnic groups) science identity work in Europe and the United States (Archer et al., 2012). Studies reveal that having opportunities to develop science knowledge and practices while being recognized for that expertise can support productive identity work (Authors, 2013). We investigated: how does co-designing and lead-teaching science classes support girls’ identity work? Does this make science learning experiences more inclusive for classmates? Theoretical Framework: We draw from social practice theory to operationalize “productive identity work” through three dimensions: deepening and integrating STEM knowledge and practices with cultural knowledge and practice, have hybrid forms of STEM expertise recognized, and using expertise meaningfully (Authors, 2013; Holland & Lave, 2009). Method: This youth participatory action research (YPAR) study (Cammarota & Fine, 2010) is embedded in a project focused on engineering for sustainable communities. Two middle school girls co-designed and taught electricity lessons to their classmates. Data includes student work, videos, and interviews. We co-analyzed the data until consensus was reached. Results: Planning and teaching supported the girls’ science identity work. They developed more science expertise as they enacted their lessons, and helped troubleshoot peers’ projects. Their class recognized their expertise. One of the girls said, “It was cool that the teacher trusted us to teach.... He was learning that kids can do this stuff too.” Both girls shared that their experience helped them see that they could, if they wanted, pursue science careers. The girls’ teaching impacted their peers’ learning by incorporating their insider knowledge of classmates and youth-culture, thus supporting others’ science identity work. For example, the girls added an educative skit because it “opened up doors to kids to show how engineering is not boring.” They disrupted how their teacher impeded peers’ identity work with a lengthy football analogy about electricity by asking an adult researcher to limit that discussion in the future. As one girl reported: Girl: [We made it less boring because] . . .We made jokes and that made it more fun. We knew the problems that kids would have, and could say it that way. Researcher: Why do you think kids like things that are made by kids better? Girl: It shows them that if other kids can do it then they can do it. Implications: This highlights how students should be positioned to co-design and teach lessons towards supporting productive identity work and more inclusive science education.

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