Background: Design thinking, with its emphasis on iterative prototyping and mantra of "fail early and often," stands in stark contrast to the typical one-and-done, failure-averse culture of the classroom. Iterative prototyping and fail-forward mindsets could promote valuable iterative practices and positive reactions to failure, but little research has examined their impact in K-12 contexts. Purpose: In two studies, we investigated the effect of a brief prototyping intervention on students' iterative knowledge, desires, and behaviors; self-reported reactions to failure; and performance on a design challenge. Design/Method: Study participants included 78 and 89 students in grades five and six, respectively. Students in an iterative prototyping condition (Prototype) were taught the process and fail-forward mindset of iterative prototyping. In a comparative, content-focused condition (Content), students focused on using science and math concepts in design. Results: In both studies, Prototype students gained greater knowledge of iterative prototyping, reported a greater desire to iterate, and engaged in more iterative behaviors on a novel, unsupported design challenge than Content students. In Study 2, students in the Prototype condition reported more positive affect and actions in reaction to failure and produced more successful designs than their Content counterparts. However, regardless of condition, students who iterated earlier created more successful designs. Conclusions: These studies provide an existence proof that instruction on the iterative prototyping process and mindset can encourage students to try early and often and promote healthier reactions to failure. This work also demonstrated a performance benefit to testing one's design early in the design process.