Suddenly class is a lame reality show.

And the richest man in the world plays substitute teacher for your little seminar:

The class burst out in laughter as the founder of Microsoft and the richest man in the United States dropped by the University of Wisconsin-Madison class unannounced as part of his tour of college campuses.

The 13 students were five minutes into a review for their exam next week when [Bill] Gates walked in.

Jaws dropped as cameras for mtvU, a 24-hour college TV network, filmed the surprise as a part of a series that features celebrity drop-ins on campuses.