The bilingual approach was both linguistically and socially misguided right from the outset, and many former bilingual teachers like me did not take long to realize its pedagogic flaws. There is no reason these children cannot retain their native language as long as they are exposed to it at home. Their Spanish vocabulary may suffer while their English one grows, but the syntactic matrixes of both languages can and will coexist, thrive individually and even reinforce and supplement each other.
Bilingualism is attainable outside the classroom, as proved in Europe and elsewhere.