That's the way I thought, right up until I took my first class. I really didn't believe there was much to be learned in a class that couldn't be learned from a book, or online.
Then I took a shooting class, and my shooting improved greatly in just two days. Hmmmmm.
I dunno what you can tell an autodidact to convince 'em that sometimes a formal class is necessary. I took my first class because my shooting buddy offered to pay for it -- and the school offered refunds to students who thought the class wasn't worthwhile.
One more point: I think you need a class to get training. Teaching you can indeed get from a book. But you need both teaching and training if you are going to shoot well.
My personal website: Cornered Cat