Most of us are taught that when something isn’t working it’s because something is wrong and it needs to be fixed. That’s not a good attitude to have when you are working with students of singing.
A much better attitude is to find out what works. Look for what a student can do not for what they can’t do. Look for things that go in a good direction, not the ones that do not. Do not fix what’s wrong, find what is right and build on it.
In order to help someone correct vocal problems or faults you have to guide the voice towards efficiency of production. That will take it away from incorrect habits and poor usage. If you tell your students the many things they do “wrong” they get very good at knowing their faults but not at understanding what they do well. Tell each vocalist about things you notice that are “plusses” and refer to the “issues” as minor things that will go away over time. Provided, of course, that you have that as a goal and know what you are doing.
If the student can’t sing high notes comfortably, work on lower notes that are comfortable and strengthen them through traditional tools: vocal freedom, posture and breathing exercises, and clear vowels and consonants. Work on vocal flexibility and strength. Work on listening for function. Done well over time the high range should emerge without undo strain as long as you incorporate trying high notes from time to time. If the student can’t sing very loudly, work on strengthening the sound from its typical level by using specific exercises, over time, gradually going from mezzo piano to mezzo forte to forte over a series of lessons.
Having a clear intention to improve something that is already functioning at a minimally acceptable level is important. As problem or issues arise, acknowledge that they are there but do not make them worse by working only on them, as if nothing else was available. Incorporate awareness of what can be better over time while expanding the domain of what is already good or excellent.
Strengthen what works, address what does not gently and in small doses. If you know what you are seeking and the student cooperates it will emerge over time.