Long time lurker, but I felt compelled to respond to this. For the record, I live on the coast of Florida and have lived through several hurricanes of varying strengths. Most notably, the hurricane season of 2004 where four storms made landfall within two months and my home was damaged and made unlivable after one of these storms.
In my opinion, the answer to this question really depends on location of the hospital, the strength of the storm, and how fast it's moving. For a lower strength storm (cat 1 or 2), if the students don't live too close to the beach and it's safe to travel (and after a cat 1 it most likely is), I think it's perfectly reasonable to ask med students to come in to help. For a cat 4 or 5, if the hospital is relatively close to the shore, probably not. This situation really has to be taken on a case by case basis. I know that the hospital near my home, which is located a ten minute drive away from the ocean, would not ask non essential staff to stay at the hospital during a hurricane lock down. However, there are no med schools near me, so I don't really know how this policy would extend to med students.
While I don't want to minimize Katrina in any way, I don't really think you can use that one very unusual experience to determine the ethics of this situation in places that are regularly hit by hurricanes. While I agree that it would be unethical for nonessential staff to stay in that situation, most storms are no where near that level of danger.