Despite the strong interest in research about collaboration among teachers, there are few longitudinal studies that have investigated improvements in collaborative activity among teachers through school-wide interventions. Drawing on data from a larger study, this article describes improvements in collaborative activity among 900 teachers at 28 Norwegian schools. The schools participated in two interventions aiming to strengthen classroom management and to prevent and reduce problem behavior. Collaborative activity was a main implementation strategy in both interventions. The impacts of both school- and teacher-level factors were investigated through latent multilevel growth curve models. Although the results indicated that teachers in both interventions improved in collaborative activity, the teachers who participated in one of the interventions improved more than teachers in the other. The teacher- and school-level factors perceived learning environment, initial collaborative activity, work experience and intervention were associated with baseline and/or growth rate results.