http://www.iceteks.com/articles.php/packetsniffers/1/

Packet sniffers, tools used to record network activity, are often controversial. In this article we look at why this is the case, and also look at positive uses for packet sniffers.

Well it's not as easy as you think. You basically need to try and trick the switch into thinking it's a hub. Never done it but I know it has to do with flooding it with a bunch of arp packets and such. But on a typical college/school network I'm sure there's security in place to avoid this, such as filtering of these packets and what not.