The most successful career - however valuable the experience can be in some respects -doesn't qualify a singer to set up as a teacher.
Ambitious pupils always feel attracted by big names. Yet it is only a few advanced pupils who mature to perfection in so-called masterclasses.
I've seen clips of such "high-level training" given by artists of the calibre of Dame Kiri Te Kanawa, Pavarotti, Kraus and many more... the advice given in such events is either correct in its purely musical character or, at the opposite, completely nonsensical when in regard to the technical side of singing. In these recordings, one can read in the mind of the poor student the frustration he feels when trying to apply the advice given without really knowing what it is that they want him to do!
What ought to matter most is the basic instruction of a pupil at the introductory and intermediate stages: this requires individual tuition by a teacher who - apart from being a singer - possesses interdisciplinary knowledge of anatomy, physiology, musical acoustics and psychology, plus of course an extremely fine ear and common sense. With the latter it is possible to detect the faults in the student and, with the former, one can connect cause to effect and come out with the solution required.
I must admit, I have witnessed many, too many singing teacher who lack the required competency. Yet they have many pupils attending their studios... In 90 per cent of the cases, these singing teachers teach at the best music institutions!!
It is no exaggeration to say that teaching singing is the least-regulated of all the professions and the saddest thing is that it seems that those responsible for the Music Staff are happy that things stay the way they are and that no formal training be given to them. What do they fear, I ask?
But don't get me wrong, there are also good teachers out there: it's just that they are not known enough.
So, when you chose your teacher to sort out your voice, my advice is: don't be fooled by their charm and popularity. Go for those things only if what you are looking for is fame and networking... you too will be able to say "I studied with such-and-such, who blablabla...", but that's another matter altogether!!