In March 2020, the COVID-19 pandemic closed all educational institutions. Teachers were called upon to respond quickly to the needs of K-12 students. They had to learn how to navigate online learning systems while simultaneously delivering engaging inquiry-based activities in high-stakes school science courses. To understand how teachers navigated these dual tensions, we have drawn on Cultural-Historical Activity Theory (CHAT) to describe how teachers learned and mediated their professional practices to meet the educational needs of their students. We examine the rapidly changing school activity system and how these changes impacted teachers’ epistemological beliefs about student engagement and evaluation. We report that teachers developed new styles and attitudes about teaching that reflected the new educational landscape imposed by the pandemic. We explore the pedagogical shifts that characterize this specific time and how the newly acquired pedagogies could find permanence in teachers’ activities post-pandemic. This study reports on the experiences of ten teachers from two high schools as they adapt to change during the global pandemic. We followed the teachers’ professional journey as they worked in a professional learning community to develop online practices. Professional learning meetings, semi-structured interviews, and participant journals captured teachers’ successes and failures as they struggled to adapt inquiry-based science lessons to meet the challenges of teaching online. Their practices shifted as they engaged students in synchronous collaborative projects and laboratory activities, and they developed alternative formative and summative assessment practices. This study contributes to a growing body of research of teacher practice through a CHAT theoretical framework to understand teachers’ professional learning during a time of change and upheaval.