And it's also the reason why throwing an immigrant in English classes won't actually assimilate them. You can't teach Levels 4 and 5 without driven students and a welcoming populace.
Learning English in school is not the same as absorbing the culture, language, and social conventions in the real-world.
I know many people that knew flawless conversational English before immigrating but they were utterly baffled by some cultural conventions and turns of phrase not covered in class.
