Consider the results of two important studies: Bacon, Kidd and Seaberg (1982) examined the eighth grade reading achievement and math scores of three groups of Cherokee children in Oklahoma: One group had had five years of bilingual (Cherokee-English) education, which included initial literacy development in Cherokee, another had four years of bilingual education, and a third group had no bilingual education. 

Without question, children in bilingual education showed no deficit. There were no differences between children in the four and five year bilingual group in reading or math,  both groups did as well as or better than the comparison group in reading, depending on whether raw or adjusted scores are used, and both bilingual groups did better than the all-English group in mathematics, regardless of the scoring method. 

Although not reported here, as far as learning Native American languages those students in bilingual education had a proficiency far beyond those without bilingual education. It is very important to note that Native Americans greatly desire that their children learn BOTH English and their Native language.

Rosier and Farella (1976) and Vorih and Rosier (1978) reported that children in an Navaho-English bilingual program at the Rock Point school had better attainment in English when compared to English-only schools on the Navaho reservation, and did better than previous cohorts at Rock Point who did not have bilingual education.The study did not utilize random assignment or attempt to control for pre-test differences. (Note that Bacon et. al. did control for a number of background factors; see their "adjusted" scores). Roiser and Farella, in fact, note that Rock Point averages had been higher than those in the other area schools since 1963-64. They were still, however, two years below national norms. 

Nevertheless, the Rock Point scores are impressive. Fifth graders in Rock Point who had bilingual education scored 5.0 in 1975 and 5.4 in 1976, compared to previous cohorts' 3.9 and 3.8. Sixth graders in 1976 scored 6.6 in reading comprehension, compared to a previous cohort's 4.7. Clearly, something good was happening at Rock Point. 

These studies are not methodologically perfect (but few are): Bacon et.al. used a small sample, and the Rock Point study did not control for previous differences. Nevertheless, it is clear that students in the bilingual programs did quite well, when compared to all-English students. For more details on Rock Point, see Jon Reyher's paper: http://jan.ucc.nau.edu/~jar/NALI7.html 

In Arizona Proposition 203 calls for the elimination of bilingual education, with little chance of reviving it - reversing 203 would require a 3/4 vote of both houses of the state government, a "super majority. Such a drastic step requires strong evidence of guilt. The results of studies done with native americans show, however, that bilingual education is not only harmless, it is, most likely, beneficial for learning English; and for learning Native American languages, it is obvious that, it far surpasses an education without bilingual education. 

Research done on bilingual programs with speakers of other languages have arrived at similar conclusions (Willig, 1985; Krashen, 1996; Greene, 1997; for a survey of studies done in Arizona, see Krashen, Park and Seldin, 2000). 
 

Greene, J. 1997. A meta-analysis of the Rossell and Baker review of bilingual education research. Bilingual Research Journal. 21(2,3): 103-122.

Krashen, S. 1996. Under Attack: The Case Against against Bilingual Education. Culver City, CA: Language Education Associates 

Rosier, P. and M. Farella. 1976. Bilngual education at Rock Point - some early results. TESOL Quarterly 10: 379-388. 

Vorih, L. and Rosier,P. 1978. Rock Point Community School: An example of a Navajo-English bilingual elmentary school program. TESOL Quarterly 12: 263-269. 

Willig, A. 1985. A meta-analysis of selected studies on the effectiveness of bilingual education. review of Educational Research 55: 269-316.