As a preface, I should say that I am a fan of the Asus Eee. It is affordable, lightweight and, for the most part, easy to use. However, that being said, personally I would not recommend an Asus Eee as a substitute for a full laptop for school.
The deciding factor, for me, is the keyboard. It is small and one has to change how one normally types. If speed is essential, as when taking lecture notes, then the small keyboard is a hinderance. The alternative is to carry a larger keyboard to use with the Asus Eee.
The problem with the additional keyboard is not only more thing to haul about during the school day. It means a balancing act between having the Asus Eee, a keyboard and perhaps a small attached-to-the-chair working space, that often is found in lecture theatres. It just makes it cumbersome.
With the possibility of four or five hours of lectures to attend during a day, working this way becomes more and more of a chore. Having a conventional sized laptop is just easier.