I taught high school for almost 10 years and I agree with both of you. You don't have to be a genius, some of the best Algebra 1 teachers I worked with couldn't teach calculus because they didn't understand it. Getting the students to understand the material is generally the easy part, their brains are highly receptive. If a student wants to learn the subject, you can basically just sit there and answer their questions. Engaging and motivating them to want to do it is the part that takes a lifetime to master.