Ok, I talked to a personel director who told me that in the event of a natural disaster, teachers are expected to remain at the school site with their students until they are relieved of the duty by the principal. He was not aware of a time limit, but he did say that anyone who left could be found guilty of job abandonment, which in terms of teacher credentialing is a pretty big deal. The teacher might very well lose their credential, and thus the ability to work in public (and many private) schools.