This paper presents the results of a school improvement study (main field test) directed at schools with mixed-age classes in the context of inservice teacher education. A quasi-experimental treatment control group investigation was designed to test the effects of the staff development program "Dealing with Mixed-age Classes," and the effects of coaching in addition to the program. Based on pre-and post-training classroom observations, a significant treatment effect was found for pupil's time-on-task levels in mixed-age classrooms and for teachers' instructional and classroom management skills. On two aspects of instructional and classroom management skills larger gains were found for coached teachers: organizing instruction and dealing with disturbances. Time-on-task levels improved more strongly in classes of coached teachers. (Author/IAH)