I have a protected Excel workbook that I send to my students, when a student
opens it with a quatro pro and then saves it, the password would be set to a
blank password this means when I receive it back from the student, I can
unprotect the sheet by just pressing tool/protection/unprotect without
entering my original or any password.

Note: I am sure that the student is not breaking the password becasue he is
failing the exam :-)

Any idea how to overcome this problem??
Thank you in advance