The National Association for Music Education . As the parent of two grown daughters, both of whom are singers, I learned several things about picking teachers and schools for them. You should be welcome to observe your child's private lessons. And they should take place at an appropriate time and place. Opportunities to perform are as important as good instruction.
Music is expensive.
Expect what you pay for sheet music to roughly equal the expense of several lessons. But it's a good investment if your child is interested. It should be good sheet music, not just what's popular this year. Singers should be taught a variety of styles and should be expected to sing in at least German and Italian as well as English. Students should be given the opportunity to compete or be rated in their area.