I have no doubt if I'd stayed there (K-6), I would have been close to fluent. After lunch was a full immersion in French. My teacher was a native speaker from Belgium. I remember having mastered counting in French, which is quite interesting considering most kindergarteners probably have not mastered counting in English.
As for an anecdote, I had one friend who stayed there through 4th grade. He still lists French as a language on LinkedIn.
[Edit] By master counting I mean having mastered the language algorithm, so to speak. Once you can count to 100, it's pretty easy to get to 200, or all the way to 999 with the knowledge at hand. It's not until one thousand that you need a new word. And, IIRC, French has a _very_ mechanical counting mechanism whereby it was even simpler. Perhaps a native speaker can weigh in here?