@LinHua ...
I think the best solution, even if they are temporary students, is to create separate user/login accounts for each student. What you've described...using an account over and over again but just changing the password...is not the most ideal, IMHO. I know you might have reasons for using a recycled account like this, but it seems to me that you're going to run into issues like you've experienced more and more. If you can come up with a system for your temporary students where they have their own accounts rather than use a more generic account, I think you're going to find these issues won't happen as often (or at all). Just my $0.02 ... for what it's worth.