School vs. In country experience
True that your writing/grammar skills will improve more in a school environment, grammar isn't everything. I had a few years in school before I first went to Russia. When I got there, the people asked me "What century did you study Russian in?" They were referring to the fact that the way I spoke and the words I used, were all antiquated. That was due to my 50-60 year old teachers that I was exposed to in the school environment. Not only were they older, but they had also been away from Russia for a number of years. They would teach us to say "groovy" and "swell" and such. We thought we knew the language when we got out of school, but the in-country experience changed all that!
The instructors always told us "They will not understand you" if you make a grammar mistake or mispronounce a word. That was also something that had me scared to death, but I quickly got over that when I realized that Russians don't use perfect grammar or pronunciation either!
So, school, plus in country experience would be best. Want to go even further, then work as a translator or learn to be an interpreter. That is when the learning really comes at you fast! Not only do you learn and understand Russian better, but you also learn a lot more about the English language.
OK, enough rambling,
Ross