I tend to want to make sure, first of all, that every one of my teachers is well-grounded in the doctrines of grace and appreciative of God's grace in their own life. Teaching ministries often attract people who feel like they have it all together (or need to be all together). And such people are inclined to teach devoid-of-grace, "what-you-must-do" lessons to others, especially to kids. This can do damage.
So perhaps the place for a teacher to start is in Galatians. Not so much for practical instruction on teaching itself, but to become better grounded as a person who lives the reality of grace in their life, resisting temptations to fall back into legalism (or teach it!). We need that kind of teacher first of all, even more than we need those who are technically gifted to teach.
Of course, that may not fit this particular teacher's spiritual condition at all. But it's true of many, and is the first thing I would check.
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