Hi,
With regard to 'Good morning, Teacher David', there are some cultural issues involved here.
First, let me say that I teach adult immigrants, so the additional problems of handling children in this respect are not part of my experience.
I believe that, in China and possibly in some other countries, all sorts of titles are commonly and widely used, eg a worker might say 'Good morning, Engineer Smith'. This is not the norm in N. American culture.
In addition, students often come from cultures in which the teacher is held in very great esteem. Thus, titles are often used as a strong mark of respect. They are often uncomfortable if asked not to do this.
I like to have an informal classroom, where everyone is on a first-name basis with me and with each other. However, I have come to see that it is tricky to ask, for example, middle-aged Koreans to suddenly be on an informal first-name basis with each other when this is counter to their native traditions. They often operate on one level with me and with each other when the 'classroom spotlight' is on them, but in quite another and more formal manner when the spotlight is not on them. After quite a few years, I haven't quite figured out how to handle this matter yet.
Best wishes, Clive