As Generative Artificial Intelligence (GenAI) increasingly permeates educational contexts, K–12 teachers grapple with concerns and uncertainties about effectively integrating these technologies to realise their pedagogical potential. This study investigates the concerns of K–12 teachers regarding GenAI integration in their teaching practices, utilising the Stages of Concern (SoC) framework from the Concerns-Based Adoption Model. We conducted an SoC questionnaire with K–12 teachers (n = 58) from an independent school in Australia to assess their concerns about GenAI adoption. Quantitative analysis shows a distinct pattern dominated by Stage 1 (Informational) and Stage 4 (Consequence) concerns, indicating teachers are simultaneously seeking foundational knowledge while contemplating educational impacts. The qualitative data expand upon the quantitative findings, revealing five key themes: GenAI as a tool for efficiency and workload relief, academic integrity issues, potential loss of the human touch, variable relevance across subjects, and specific professional learning needs. Drawing on these insights, we propose a targeted approach to teacher professional development (TPD) that begins with GenAI literacy and ethical considerations, then moves through pedagogical application, practical implementation, and collaborative learning, ultimately creating space for innovation and transformative practice. This approach allows professional learning to unfold with a developmentally appropriate focus, optimising resources while addressing teachers’ varied readiness levels and evolving concerns. In doing so, the study contributes both to theoretical understanding of GenAI integration in education and to the design of responsive, teacher-informed professional development strategies.