Unraveling how children become bilingual so easily

I remember a while ago there was a debate raging about how California public schools should deal with young ESL children. Should they immerse them in English-only courses (good for English-learning, bad for 'everything else'-learning and putting them at a disadvantage) or should they start in Spanish and slowly transition them to English-only courses? It was an interesting discussion, but was (understandably) framed around the idea that the children were part of a growing problem which badly needed a solution.

Simultaneously, I (a native English speaker) was sitting in a required high school Spanish course (two years), which was progressing at turtle-like speeds and effectively useless unless you were in AP Spanish.

For the life of me I can't understand how these two 'problems' can exist in a school system and administrators can't put two and two together. Now at 28 I'm actually learning a second language, and I can see both how useless those high school courses were, and how much I would have benefited had I learned a language when young, while surrounded by other children who were 'native' speakers of that language.

My kids will most definitely be raised bilingual, and I hope to expose them to a school which facilitates language learning from the get go.