The more I dance with my practice partners and the more I see that they improve with every dance and the dramatic difference between what they can do after a month and what the general class-taker can do.... has led me to believe that Chrisjjjj is right; dancers learn best by dancing with more advanced dancers...
In another thread, a few days ago, you presented your "practice" plan, which turned out to be more like a teaching plan. When you practice with your partners, do you offer advice? Do you not consider that to be teaching them?
Sure, there are good teachers and bad, just like every other human occupation. Sure, private practice sessions/lessons are better than classes, but that doesn't make classes bad.
I've known plenty of dancers who never took classes seriously or diligently and expected to learn by just dancing. Most of them were not adept enough to do that, and now they either no longer attend milongas or they spend lots of time sitting, because they haven't improved very much.