Project-based learning has been suggested as an effective student-centered practice to teach science. However, how to assess students' learning in project-based learning activities remains challenging because of its open-ended nature. In this study of high school students' science internships, we demonstrated that "cogenerative dialogs" can serve as a formative assessment to improve teaching and learning in project-based learning activities. Cogenerative dialogs provided a safe and supportive space for respectful and equitable dialogs. In cogenerative dialogs, both instructors and students identified areas for improvements and brainstormed solutions as a team. As a result, students were empowered to speak up their voices and willing to take on the responsibility for their learning. The regular cogenerative dialogs became an effective form of formative assessments to improve both the teaching and learning involved in the science internships. By analyzing these cogenerative dialogs, we identified eight key strategies used by participants in cogenerative dialogs to justify and assess the internship teaching and learning. These key strategies may serve as a framework to observe how instruction revolutions and feedback are intertwined and how students become engaged in assessing themselves and their peers in formative assessments.