Given that grouping students homogeneously has proved to be more deleterious than beneficial to student learning and self-concept, principals of middle level schools have the responsibility to implement more heterogeneous grouping arrangements in their schools. Grouping programs that provide multiple criteria for determining student ability and adequate guidance for students selecting courses and that do not lend themselves to a schoolwide grouping pattern are more likely to meet the variety and multiplicity of early adolescents' intellectual and developmental needs. The successful implementation and continuation of more heterogeneous grouping arrangements in any middle level school hinges on the principal's ability to identify the short-range and long-range skills necessary for the full realization of the new program and to provide staff members with adequate, ongoing training in those skill areas. Cooperative learning has emerged as one effective and constructive instructional strategy applicable to heterogeneous grouping arrangements, yet teachers will need extensive education and skill development in this area if students are to reap the anticipated benefits. The first step toward a successful heterogeneous grouping program in the middle level school is for principals and staff to collaborate and develop a timeline for the provision of necessary instructional skills. (13 references) (CLA)