The lens a teacher sees through is important. There are those who can look at something and say, "I see it", while others struggle, similar to a hidden picture where one stares at a design's backdrop until the appropriate image comes into focus. The combination of training and experience relative to the field a teacher is providing guidance in is absolutely necessary for them to have a proper perspective of what and how to teach. Students come in all forms, with many variances in strengths and weaknesses. Teachers should be able to see in students what others may miss, and identify these innate characteristics. In music, they are to help students discover themselves, discover their sound, and develop both specific and overall abilities maximizing the student's potential.
Using standard method books, repertoire, and general teaching resources is a given for any music teacher, however supplementing understanding beyond this point is important. Anybody can point to a page of instructions and read from it, or open a lesson book and merely monitor as a student guides his or herself through it. In fact, the best test of a truly capable teacher is one who can teach with no external resources at all. Those that can present ideas and connect concepts using only themselves as a resource, taking combinations of technique, theory, application, experience, and solid understanding to produce successful students, demonstrates true teaching skills in the rawest form. Teaching is more than colorful books, and reading notes off a page. It is being able to identify potential abilities in an individual, and conditioning that student through a form that allows these abilities to surface and become visible.
A student can never reach his or her full potential in music if they're being taught in a way that doesn't coordinate with how they've developed on a mental, physical, and artistic level. With vocalists, the physical vocal build of a student is important in regards to what they study and how they study. Just like an athlete's innate physique is appropriately tuned for a particular sport, a singer's vocal makeup carries properties specific to fields like Classical, Jazz, Commercial, Pop, Rock, Musical Theatre, etc. Though students can effectively pursue whatever interests them, it is foundational that they learn the natural characteristics to their own voice, as well as the proper mechanics behind vocalizing. On a mental level, students also have to be fed information they can properly digest, and just like a hidden picture, it's up to the teacher to see all of these things and instruct accordingly.
In addition to the training from various teachers and mentors I've been fortunate enough to have had, the larger majority of my musical experience has been built on performing. As great of an attribute as being labeled a "performer" is, it by no means equals an automatic pass into being a great teacher. This stands as a hot topic amongst many singers, musicians, and those striving for success in the teaching field. We are all artists and passionate ones at that, however teaching is it's own animal with its own communicative requirements and comprehensive skill set, and it has always seemed to me that those truly dedicated to the craft agree. To see the hidden image within a picture requires the appropriate perspective, as does being able to hear music within the chaos of noise. Teachers come in all varieties. Those with the ability to see the hidden talents within students, and challenge students in a way that naturally unlocks these talents, prove to be without a doubt the most effective and capable.