Doug Fattic in Niles, MI offers classes that I'm thinking about taking. Brew Racing, NC offers classes, as well as Yamaguchi in Rifle, CO and the UBI courses in OR. Apologies to any I left out.
As I heard often in (law) school, "Pay your money and take your chances."
No matter how much you learn in a frame class, you'll be scratching the surface, but you'll wind up with a frame. Not aware of any real apprentice programs. Most solo builders are too busy building to have to go in front of and behind somebody else all day. There are some smallish companies where a person might get a toehold and learn bunches, but I'd be prepared to sweep floors for a while...GOTTA love it.