I bought a film camera recently and it reminded me of my first camera, a present. My father sat me down and explained photography, explained the camera, the film, and how it should be used. I was about 11. But when I think about it I realise that he didn't teach me how to make photographs. That was something I had to learn by myself – and am still learning.
You look at photos, you read about them, you talk to other photo makers about their photos and yours. Even now, it still feels like an endless process, as if you take/make photos in order to find out how to make them, how to make them better. (Or 'take' them: the one real mentor I had in photography spoke French, and she always spoke of 'making' photos.) & if you put everything into it you might end up making photos that can be recognised as yours, and no one elses's.
I think it may be some kind of category error to assume that teaching or learning a practical, creative activity like dancing, painting, photography, is the same process as teaching or learning a theoretical or factual subject like history. I think very few of us were encouraged to learn by enquiry: we were passive containers to be filled with knowledge. So, now, if we want to know something, immediately we become students, all ears for our all-knowing teachers. It's a familiar way of learning.
But approaching photography or dancing this way really doesn't make a lot of sense. Sure, there's a certain amount of basic stuff you need to be taught in painting, but you can't solemnly file into a classroom week after week to be taught to paint: you have to go out and buy some tubes of colour and make a mess, and keep making a mess until you start to find something that makes sense to you and others. The same with tango. You've got to be released into the wild pretty early on to develop survival skills. A teacher can, and should, explain the skills you need, and demonstrate what you need to do, but you've got to go out there and join the community, learn by trial and error. & hopefully some wise old teachers, sympathetic partners, perhaps, who've been round the floor more than a few times, will be there to help you improve your skills and develop your musicality.'No! Put your foot there, not here!' or 'Con el cuerpo!', as Pedro keeps saying; dance with your whole body rather than just with your feet.
There's a certain amount in tango that does need to be taught in classes of some kind, whether one-to-one, or in groups, and these days there are so many people who want to approach tango that group classes are the most feasible option. But it should be clear from the start that the real learning is done on and around the floor, preferably in practicas, by trying and by watching. That is learning, it's an active process, an activity, and it covers a whole area that simply can't be taught.
As I've mentioned before, there's a real shortage of practicas in London. Chris left a link to a list of Berlin practicas, and as far as I can make out there are at least two every night, more some nights. In many ways I think practicas suit our relatively informal society better than milongas.