There is a point of view that says you should use songs to teach vocal technique. I do not agree with this.
Vocal exercises are the correct means of developing vocal technique. It is the exercises that allow you to develop all the capacities of vocal skills that are necessary to creating vocal mastery. The list of things that a well developed voice does is specific and rather long. I have written about it here in the past and won’t repeat it now.
Most teachers of singing choose songs to help the student master various vocal behaviors. This is bound to cause problems. Generally, songs lag behind vocal exercises by quite a bit, weeks or months. Therefore, teachers of singing should make sure that the song is easier than the exercises, well within the scope of the vocalists “cruising” ability. This is the only way to teach performance. If you can’t get through the phrase because it is too loud, too long, too high or low, or because it is musically very hard or has lots of words sung quickly, working on the song will only cause you frustration. It will also interfere with your ability to feel and express the meaning of the lyrics or poetry. You will not be able to concentrate on what the songs means to you if you have to concentrate very hard on making your throat do something it does not yet, on its own, do. This is sad, and is a very typical situation which causes great confusion because, inevitably, the student is blamed.
If you want a student to “learn” mix and you choose a song that is mostly mid-range in terms of pitch you MIGHT get the student to be better at mix but you might also wonder why the student just keeps flipping from chest to head, over and over, and never seems to get any better. It would be because you have not yet gotten the student’s vocal mechanism to be established strongly enough in mix for mix to do its job automatically. If you pick a belty song with the idea that this is somehow likely to help the student develop mix, asking the student to sing in a vocal quality that he or she will not find in a professional recording (as an aural reference), you will simply confuse the student. Of course, if you do not even know the song is meant to be belty because you didn’t bother to listen to it first yourself, you should be boiled in oil, but you already know that if you read my posts.
Teach technique to develop technique. Use exercises to develop vocal and breathing coordination and skills. Teach performance in songs (acting, movement, stance, etc.). Do not confuse the two.