Which research question would be the most effective for researching bilingual education in elementary school?

how many elementary school students were raised in bilingual households?
what languages are taught in elementary schools around the country?
which language would be the most enjoyable for elementary school students to learn?
how does learning another language affect elementary school students’ overall academic success?

The research question that would be the most effective for researching bilingual education in elementary school is that last one - How does learning another language affect elementary school students' overall academic success?
This would be the most important research question to ask because valuating students' success is quite relevant, and it is especially interesting to see how these students cope with having to speak and learn in two languages, both of which are supposed to be their native tongues.