What teachers do in the classroom is a key determinant of student learning processes and outcomes. The current study explores the levels of and improvement in teachers' instructional support throughout a school-wide initiative that aimed to develop classroom management in Norwegian lower secondary schools. The sample contained 227 teachers from nine schools. The results of growth mixture modeling (GMM) indicate that the majority of teachers did not improve. The teachers who did improve reported higher initial instructional support levels than the non-improving teachers. The improving teachers also reported higher job satisfaction and less stress from student behavior. No significant differences were found between the two groups with respect to emotional exhaustion and stress from workload. The study illustrates an approach relevant for evaluating improvement in teachers' practice within specific interventions as well as in ongoing professional development. The results may have implications for the length, focus and content of professional development for teachers. The study provides knowledge that may be relevant to school and district leaders initiating improvement efforts in their school(s).