Partnerships between researchers and educators have become a common mode of working to support processes of school improvement. However, research on the processes and outcomes of long-term partnerships remains marginal. This paper increases our knowledge about the processes and experiences involved when researcher-interventionists introduce theoretical models as tools to facilitate reflection and change in long-term partnerships with upper secondary school leadership teams when working on specific challenges and dilemmas. Through an analysis of 41 hours of video data from Change Laboratories and 11 interviews with three upper secondary leadership teams in Norway, the paper examines what unfolds in such research-practice partnerships. The analysis is informed by cultural historical activity theory, with a focus on tools and mediation. The findings suggest that the introduction of theoretical models mediated the sharing of information and alignment of opinions and experiences, identifying issues and altering work procedures, but the participants found it challenging to utilise theoretical models perceived as more general and conceptual compared to those they regarded as organisation-specific and directly applicable. The paper argues that theoretical models may function as bridges to development by clarifying the object and stimulating change in research-practice partnerships, but to do so, researchers need to align the presentations of such models to context and timing.