“But I marvel when I observe these men setting themselves up as instructors of youth who cannot see that they are applying the analogy of an art with hard and fast rules to a creative process.” (Isocrates)

So teachers of rhetoric need to be flexible, and to really understand their subject. If they don’t know what they’re talking about, surely, they are unfit to teach.

“…and the teacher, for his part, must so expound the principles of the art with the utmost possible exactness as to leave out nothing that can be taught, and, for the rest, he must in himself set such an example of oratory that the students who have taken form under his instruction and are able to pattern after him will, from the outset, show in their speaking a degree of grace and charm which is not found in others.”

Again, a teacher should be an expert, and a teacher must be thorough. Moreover a teacher must be accomplished in the art that he professes to teach. It’s very well and good to have all sorts of knowledge about your subject, to get the theory of it, but they better also provide an example of how things should be done.