Since the issuing of a visa would require a letter from an accredited university offering the student a course, which somebody has to pay for (often their parents or government but not, for once, us) then the visa would be for the duration of the course. There is no need for an extension surely. Once the course is finished, and the student will well know when that date is, they can leave. It's not as if it is a last minute thing. They have, after all, three years to think about it plus all the "finals" etc to go through so they know well enough when their visa is due to expire.
Mrs Snoopy started on a post graduate course in October 2006 ~ she knows and has known all along, that it will end on 26th May 2008. She also knows that if she wishes to take another course or perhaps a PHD to follow on from the current course she must apply for a place at least 6 months before it is due to commence. The plain fact is that these "students" never had any intention of going home at the end of their studies.
