The answer will probably surprise you. It is (in general) harder to teach a beginner than it is to guide an advanced student! Why is that? Because a beginner will ask (in his or her naiveté) BIG QUESTIONS, questions that defy those who do not understand the basics well. You have to know MORE to teach a beginner, because you can always hide behind complexity, but there is no pat answer to such a question as "Why does the Treble Clef look the way it does? Why is it on that particular line?" Badly taught teachers often disguise their ignorance behind a cloak of pretended knowledge or altitude or tell you "Just do it."
Perhaps there is one exception to the rule that beginners are harder to teach and demand more knowledge than teaching advanced students--and that is the "advanced" student who has learned all the wrong technique or who is missing whole sections of basic understanding. When a student seems to dazzle at the keyboard, but holds arms and wrists in such an uncomfortable manner that he or she could develop searing pains and even need operations later on, well THAT is the most difficult student of ALL to teach. Unlearning bad habits can be an ordeal, but it is an ordeal well worth it in the long run.
