How would Yoda teach all of the younglings? By the looks and size of the Jedi Temple, they would have loads of younglings to teach. Yoda was the head of the Jedi Council, he instructed all of the Jedi to do certain jobs. The other council members give suggestions and stuff like that. So when Obi-wan said that Yoda was the one who instructed him, he meant that Yoda was the one who told him what to do, not teach him to be a Jedi.
Yoda only instructed them in certain things though, they also had lightsaber trainers and stuff.
And I read somewhere that Qui-Gon actually found Obi-Wan as his padawan when Obi-Wan failed as a youngling or something and was put in the 'something' Corps, a kind of workforce that helps the poor on certain places. I think it was because he was too angry/ rebellious or he tried to hard. This is supported by the line in Empire 'was i any different when you taught me'