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