Actually, this is a very interesting thread.
I've lectured first year, third year and final year students over a long period of time

. During that time I've also been through the dancing learning phase (from beginner up to competition pre-champ level)and i 've leant a lot about my teaching/lecturing skills from my experiences learning to dance.
I've thought about this 'un-learning' situation a lot because i've had the same thoughts as drscience.
The way i currently view this is as follows:
1)You can't teach 4th-year material to 1st-year students
2)Understanding of new information is built on previous knowledge and experience, and the better this previous knowledge, the faster you will learn new material. So very good basics are important.
3)Never knowingly teach anything which is wrong or bad technique.
So for example, you would go to some trouble to make sure a beginner closes feet correctly in slow waltz (where it's needed of course), keeps good timing and avoids poor posture. These are serious fundamentals.
Competition dancing is an adaptation of good social dancing but the basics are the same.