Some teachers of metaphysics claim that they examine the Ba Zi of a potential student before acceptance or rejection into their schools. Mainly this is to avoid betrayal. Some will base on Mian Xiang (face reading) to accept or reject a student.
Confucius said that he offered his teachings to anyone including those who merely gave him some preserved meat as a token of thanks. Well, the preserved meat was only symbolic. In other words, he taught anyone who respected him as a teacher. He taught a lot of students but only 72 followed him through to have great achievement.
I have quite a number of good students. The most comforting thing is that none of them brags about being knowledgeable. There are others who claim to be the best ... There are some others who step on successful people in order to gain recognition. They simply lose their credibility.
We have witnessed some frogs in the well bragging that their bellies are larger than the sky. In order to prove their claims, they inflated and busted their bellies.