The study reported in this article analyzed the length of time required for 1,548 advantaged limited English proficient (LEP) students to become proficient in English for academic purposes while receiving instruction in English in all subject areas. Variables included were age on arrival, English proficiency level upon arrival, basic literacy and math skills in the native language upon arrival, and number of years of schooling in English. Second language and content‐area achievement were measured by students' performance on the Science Research Associates tests in reading, language arts, mathematics, science, and social studies. The results indicated that LEP students who entered the ESL program at ages 8–11 were the fastest achievers, requiring 2–5 years to reach the 50th percentile on national norms in all the subject areas tested. LEP students who entered the program at ages 5–7 were 1–3 years behind the performance level of their LEP peers who entered the program at ages 8–11, when both groups had the same length of residence. Arrivals at ages 12–15 experienced the greatest difficulty and were projected to require as much as 6–8 years to reach grade‐level norms in academic achievement when schooled all in the second language. Whereas some groups may reach proficiency in some subjects in as little as 2 years, it is projected that at least 4–8 years may be required for all ages of LEP students to reach national grade‐level norms of native speakers in all subject areas of language and academic achievement, as measured on standardized tests.

To expand the current theoretical base in second language acquisition, this article proposes nine generalizations on optimal age, L1 cognitive development, and L2 academic achievement. These generalizations summarize the author's and others' research on second language acquisition for schooling purposes. In this synthesis, relationships among the following variables are considered: first language acquisition, second language acquisition, student age at the time of exposure to a second language, academic achievement (as measured by standardized tests in all subject areas), membership in a language majority or language minority community, and language(s) of instruction in school. The five new generalizations presented at the end of the article, which are based on research on academic achievement in a second language, merit additional research to validate and refine them.

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This chapter summarizes the findings of 32 years of research from all of our longitudinal studies to date, conducted in 36 school districts in 16 U.S. states, more than 7.5 million student records analyzed, following English learners (of all language backgrounds) as far as Grades K–12. These studies are very generalizable to all regions and contexts of the United States and have been replicated in other countries, answering questions regarding program effectiveness for policymakers in education. We have shown that English-only and transitional bilingual programs of short duration only close about half of the achievement gap between English learners and native English speakers, while high-quality, long-term bilingual programs close all of the gap after 5–6 years of schooling through the students’ first and second languages (L1 and L2). In addition, our studies answer the linguistic question of how long it takes student groups to reach grade-level achievement in their L2, and we have developed and refined our theoretical Prism model by collecting and analyzing program effectiveness data, basing the Prism model on our empirical findings.

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