Teachers in my school often screen students - not necessarily when they ask to join the class, but as an ongoing process. My sifu often warns us to keep an eye on so-and-so, sometimes even tells us not to show them anything that might be dangerous.
My late Sigung was known to refuse students if he felt their character was no good.
Often both my sifu and sigung would take in a student with questionable character to try and change them. I guess they were still careful in what they taught these students until they could be sure that the student had changed. Personally, I do not have enough confidence in my ability to influence someone to try something like that .
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Behold, I see my father and mother.
I see all my dead relatives seated.
I see my master seated in Paradise and Paradise is beautiful and green; with him are men and boy servants.
He calls me. Take me to him.