I am already completely disheartened at the leftist push toward bilingual instruction versus English immersion, with not even a pretense of balanced research to evaluate both methods.
According to the gurus at SIT (and they are considered the best in the world in this field), English immersion is the most effective. In fact, the best situation is a class in which none of the students speak each others' language! (For example, one student is a Spanish-speaker, one a Chinese-speaker, etc., so they can't even talk to each other without using English.) Big companies pay big bucks to send their executives there to learn English with this type of class structure because it really is the most effective.
On a practical note, binlingual education might be the best route for older children of workers who plan to return to their home country, as learning content is more important in such case than learning the language. Otherwise, bilingual education is for folks who can afford to educate their children to be truly bilingual and who aren't themselves fluent in two languages (like the parents who send their kids to those private New England schools.)
One concept of bilingual education is that ALL students learn BOTH languages and are immersed in BOTH (as at those fancy private schools). What ends up happening in our public schools is that only the kids who need to learn English are given bilingual education and we born-here Americans still can't really speak anything but English very well.
Well, I hope this was helpful.