This study aimed to identify the factors that foster a collaborative culture in the school improvement process. Estonian schools are characterized by a high degree of autonomy in developing the school curriculum and choosing the appropriate methods for its implementation. As a result, some schools are more successful, while others face difficulties in improving their pedagogical processes. Six Estonian schools with lower performance indicators participated in the School Improvement Program in 2021-2022. In each school, leaders and teachers formed a team together with two mentors. Supported by university experts, the school teams began working on a topic they chose to improve their students' learning while simultaneously increasing the school's leadership capacity by strengthening a collaborative school culture. The results of this qualitative research demonstrate that the arrangement of teamwork and the creation of shared values and goals constitute the key factors in creating a collaborative culture. Collaborative culture can be fostered by composing a stable team, developing routines for collaboration, ensuring open communication among all parties, focusing consistently on the goal, and building trust among participants. The obstacles are resistance to change, an unstable team, no routines for collaboration, a lack of communication, and no commitment to the goal. External support is important for both successful school teams and those facing challenges in the improvement process.