Here's an article to help teachers teach proper head and shoulder placement in class - "How to Use Correct Placement in the Head and Neck for Dancers". Another reference is a previous post, here on Dance Nerds Unite - "Improving Port De Bras" http://dancenerds.blogspot.com/2010/10/improving-port-de-bras.html
The four main bad habits the author says dancers make with their heads are:
- Holding tension in or straining the neck
- Focusing the eyes in a different direction than the head
- Jutting the chin, or lifting it too far
- Throwing the head, rather than controlling movement
Check out the article at the link above for more info!