Stipulating that you have more legal experience than I do, I still have to question the assertion that showing the Facebook page while logged in is functionally equivalent to granting them the password.
In the latter case, the school official would have unfettered access to her page until such time as she was able to change the password. As a practical matter, this would, for the majority of the population, also mean that the official would have access to many of her other online accounts (the vast majority of people use the same password over and over again). This just isn't the same thing as her sitting at the keyboard while the official looks over her shoulder. If she's the one sitting at the keyboard "driving" she need not reveal the actual password, and she can log out immediately rather than trusting the school official to do so later.
I'm not sure that the distinction will be meaningful within the set of facts at hand in this case, but I can definitely see reasons for differentiating between the two actions (if only insofar as deeming a demand for the password itself as a greater offense).
ZV