While performance assessment (PA) is well aligned to project-based learning (PjBL), teachers find it challenging to design and implement PA that is faithful to the authentic context of their projects and viewed externally as rigorous. In contrast to standardizing PA tasks--thereby diminishing authenticity--we formed a research-practice partnership (Coburn, Penuel, & Geil, 2013) that developed and used a "shell" to guide teachers in planning, implementing, and engaging in rigorous dialogues that evaluate and elevate PA practice across four PjBL schools. Drawing from analysis of artifacts and audio-recorded professional development sessions, we highlight how the effort to standardize PA practice while maintaining fidelity to authentic context provided rich opportunities for teacher learning and fostered higher levels of teacher responsibility for assessment.