This publication presents a summary of an ongoing collaborative research study that is both national in scope and practical for immediate local decision making in schools. It is written for bilingual and English as a Second Language program coordinators and local school policy makers. The research includes findings from five large urban and suburban school districts in various regions of the United States where large numbers of language minority students attend public schools, with over 700,000 language minority student records collected from 1982-1996. A developmental model of language acquisition for school is explained and validated by the data analyses. The model and findings from this study make predictions about long-term student achievement as a result of a variety of instructional practices. Instructions are provided for replicating this study and validating the findings in local school systems. General policy recommendations and specific action recommendations are provided for decision makers in schools. (Contains 71 references.) (Author/SM)