This post a few months old I know, but what the heck, let's take a stab at it.
To my knowledge, that's not a scripture verse. There is a verse, 1Cor 11:1, Paul says "Imitate me, as I imitate Christ", but that's different than what you're getting at. There's Proverbs, "Lead up a child in the way they should go, and they'll continue to walk in it" but that's also a bit different. There are probably verse sin the prophets that say that leaders are leading people astray via their bad examples. But as for a verse exactly like what you mentioned, in relation to peter or an apostle in the NT, I don't think it's there.
But now we get to the real question:
I think you meant "unrepentant sinner".
We all have sinned and fallen short of His glory, and we are exhorted to continually ask for forgiveness and to profess our sins, which tells me that in our Christian walk we will sin. But there are passages in the Bible, say the passages about Jesus saying, you'll know a tree by it's fruit, the passages in 2Peter and Jude about false teachers, and the scary Hebrews 6 passage, which does seem to point to professing Christians who's lives really don't live up to the what they are professing (ie being a Christian). Also, Hebrews talks about drifting away, don't drift away or loose the faith, remain fired up like you where at first; and 1Jn talks a lot about those who practice sin don't really love God, the phrase "practice sin" meaning willfully habitual, a lifestyle. So yes, while we Christians do all sin, it seems that there is a way to live where, you profess to be a Christian but really aren't one. If we knew someone was on that side, then I'd say we're correct to not let them be teachers and leaders- besides the fact that they aren't truly Christians anyway, children especially are very prone to following examples.
How would we "judge" this? Well first we'd take the planks from our eyes. Then, we'd do the same that we'd do for almost anything- pray about the teachers; have a good grasp of what scriptures say is a Christian; discuss this topic with other mature, Godly Christian adults; when a specific teacher comes to the forefront, pray and study and discuss, and eventually the pastor would have to confront the teacher if it leads to this.
Overall, yes it'd be good to not have Christian who are merely professing but really are not the real deal, be teachers in any capacity. They should definitely have repute and wisdom, and I'd say they should have the gift of teaching. If God gives gifts, why are we running around begging for and taking any and every volunteer we can get our hands on? God'll provide.
