Recently published results from a 5-year randomized study indicate that Spanish-speaking children learn to read English equally well regardless of whether they are taught primarily in English or in both English and their native language. The first of its kind, the study compares English and Spanish language / reading performance of Spanish-dominant children who, from kindergarten, were randomly assigned to Transitional Bilingual Education or Sheltered English Immersion.
Slavin, R.E., Madden, N., Calderon, M., Chamberlain, A., & Hennessy, M. (2010). Reading and language outcomes of a five-year randomized evaluation of transitional bilingual education. Baltimore, MD: Johns Hopkins University.