[drewcoo]

Presumably this depth of understanding should be deeper than the students. And probably a little deeper than the material taught. That way the teacher can inform and maybe inspire students to study further.

How much depth is required to have a deeper understanding of anything than a 7th grader? How about 2nd grade?

[pbuzbee]

I think that falls back to (A) in my original post. You certainly don't need a Master's in pure mathematics to teach 3rd graders how to add fractions, but you probably do want to understand what teaching techniques are effective for children that age.