To go back to the OP, classes are not the only way to become proficient. But they are a very good way and as others have pointed out, coaching is even better.
I had some kind of coaching or training weekly if not daily starting when I was eight or nine years old and continuing until I was about 15. That's what happens when your father is on a National Guard shooting team and is deadly serious about being good at it.
I was also an active-duty Special Forces soldier for several years during the Cold War. I got almost no "training" on shooting in the sense that we're using that word on this thread. (On a handful of occasions, one of us would put something together on one of the less-structured range days that were all too rare back then. Sometimes that was very good. On a couple of occasions it was horrible and misinformed to the point of being counterproductive.) What I did get was a ton of training on how to fight: how to apply shooting skills in dynamic situations, how to plan your way in and out of them, and how to work in accord with other armed people.
Since I left the Army, I've trained with some solid folks. The best taught me how to continue training myself after class was over. For instance, Pat McNamara, Ken Hackathorn, and Larry Vickers would call it out in class: "Hey, guys, here's how to stay proficient and get better after you leave here..." Then they'd explain how to adjust par times, conditions, and scores for standard drills to improve specific aspects of performance. Dagga Boy also talks about this in the threads on the D Platoon Qualification Courses. I think that a lot of my fellow students missed the importance of this because it came late in the class when we had been shooting for several hours and were too focused on range-level minutiae and loading mags to recognize an actual long-term strategy that could keep us growing and learning as shooters/fighters for years.
That said, a lot of it depends on what you want to do with the skills you acquire.
6 will win a lot of fights but not all of them. 8 is a better place to be, and we all need help to get there.
Okie John