As my previous post touted the private Denver International School, it's only fitting that I also promote Lafayette, Colorado's own Pioneer Bilingual School, part of the Boulder Valley School District. As I understand it, at Pioneer all teachers speak both English and Spanish, but teach exclusively in one language and are paired with another teacher who teaches in the other. Students alternate between the two and thus receive all their science, math, social studies, and so forth in both languages. The school's student ratio is 50% native Spanish speakers and 50% native English speakers.
In my volunteer tutoring at the Lafayette Public Library Homework Center, I've worked with many kids from Pioneer and am regularly humbled by their ability to read a book in Spanish and write a book report about it in English after completing word problems for math class in English and explaining why leaves are green in Spanish--in third grade!