Fortunately, we've had only one serious incident of a student deleting other students' files. But we use that as an example in our training session to remind teachers that students are responsible for keeping their passwords secure. We also recommend that students keep backups of their own files just in case.
Establishing clear groundrules will reduce the number of annoying interruptions teachers might face in class, for example, telling students that they should read their email but to read it before or after class. Given the amount of self-paced work students do, most of our teachers have found that they can have a quiet conversation when they see a student spending class time on activities that don't belong in the class.