An Obvious Blemish

Rav Dovid of Skver, zt”l, explained how to ensure that one’s children grow up pure. “The main thing in chinuch is the quality of outside influences on one's children, especially their friends. This is what my father zt”l, would emphasize: only if there is a genuine 'סור מרע' can there be continuity to the children’s 'עשה טוב'. The way we fulfill 'סור מרע' with children is to protect them from bad friends, because they have a huge influence, especially while one is still young and easily swayed.
“My father even recounted that one of the great tzaddikim of his generation worked very hard to educate his children to yiras shomayim but was only partially successful since he was not discriminating about his children’s friends.”
The Rama, zt”l, learned a similar lesson from a halacha regarding halachic leprosy. “Our sages teach a paradoxical-seeming halachah. Although if one contracted צרעת that is larger than the size of a גריס he is defiled, if the צרעת spreads over his entire head or body, the leprosy no longer defiles and does not require quarantine.
“This halachah teaches us a very essential lesson about impurity. It is only when the impurity is not readily obvious that the impure can have an influence on the unwary or naïve. When the person only seems to be slightly blemished, one is prone to learn from his negative actions, rationalizing that he must not be so bad. But if his impurity is readily apparent, everyone knows that he is wicked and acts out of wickedness. Since no one will copy his evil ways there is no longer any reason to quarantine him!”