Found in proverbial truisms since for thousands of years, it is a proven fact you cannot exceed the knowledge of the one teaching you (you can get another teacher - but that is a different aspect)
Any given teacher can only impart what [s]he knows (or thinks)
So what does the 'smartest' or 'most-knowledgeable' AI actually look like?
I disagree. This is not how knowledge works.
You can learn new things yourself. When under instruction, you may notice nuance your teacher has not mentioned, and may be unaware of. There is also learning without instruction.
Call that "self teaching" maybe? Saves your aphorism but it still a strawman question.
For an AI to be self-learning, it will have to have a lot more senses and opportunity to manipulate and observe the world on its own, than anyone seems to want to give them.