I was thinking about this last night. If you were to choose between the 2, which would be the better option to drastically improve your skills in a language?
Foreign country - You'll probably struggle at first especially when there's no one to teach you the technical side of the language. But you'll eventually get the language via osmosis and constant practice.
School study - Gradual progression is a good benefit when you learn in a class. However relying too much on books can be detrimental as there are cases where guys who learn solely from books sound "unnatural" in the way they express themselves.