> Isn't it better to have an expert who is a mediocre teacher than an excellent teacher with only a mediocre command of the subject matter?

Suppose we suppose to suppose.

An expert visiting a classroom with a competent teacher would be the best thing -- the teacher can provide direction and control over the class as well as assist with prep.

A Boeing machinist specializing in door plugs might make for a dry presentation. But a Boeing machinist specializing in door plugs who has the help of a good physics/engineering class teacher to lay out a lesson plan would probably be pretty fun.

It depends on the student being taught. Most (HS) students will do better with the excellent teacher than the subject matter expert. Perhaps what you actually want is for unusually bright or creative students to have access to experts? Maybe some sort of opportunity to observe and even help them in their field? Perhaps we could call it something that doesn’t sound anything like the activity in question, something like “interning.”

That's a very difficult question to answer - but I'd say, "no", especially for the younger grades.

Most of what's being taught is how to learn - and knowing how to teach is the hard part.

I think it depends on the how advanced the class is. An excellent teacher with mediocre subject matter expertise will outperform a subject matter expert with mediocre teaching skills in all but pretty advanced courses.

Also, my experience is that most experts aren't mediocre teachers, they're terrible teachers.

Why not pay the money to get excellent teachers with a good command of the subject matter?