Some pedagogically useful example drawings by myself, and also works by some of my students.
Please note that the drawings by my students are not for you to criticize/judge. Some of them are mere little ones (around 9-11 years old), who are brave enough to take the first steps to learn drawing properly. If they stay the course, they will far supersede all of us living today. It's just like philosophy or wisdom- the younger they begin on the right path, the further they will go, the more insights they will make on their own. It is our duty to light the way for the future generations, and not repeat the mistakes of our present generation.
I will roughly categorize the works into sub-headers as I think it useful.
Some of them struggle more than others, of course, but that's where the learning actually begins. If one falls back to just "lazy" naturalistic copying, one can draw for years without any understanding, thereby affecting the creative output further down the line, because of the lack of mastery of form. I stress that it is very important to always consider the unseen planes when drawing the ones that are seen, to examine the relationship between a form's elevation and plan.
N.B. This is by one of my most talented young student(11 years old) who did this from life in about 2 classes. I don't correct my students' works in most cases, usually I draw in the margins such as shown here.