This is such a bad idea it deserves its own post. One of the problems with online authentication is that a person could sign in to a test and then hand it over to someone else to complete, thus defeating the login. The solution proposed here (22 page PDF) is to collect information about the student during the course and then to pose periodic authentication questions during the test. According to this paper, the method works. I'm sure it does. But it's a solution that manages to be intrusive and annoying at the same time!