I'm not sure I agree with SAS's assertion that "To be a good teacher, you need significantly more knowledge than the level you are trying to impart..."
I would say you need to know the level you are trying to teach REALLY WELL, with enough knowledge to answer any subsidiary questions the student is likely to ask.
When your student might be a PPL who will never fly a twin, let alone an airliner, do you need to know about the errors of amachmeter?
tp