My cousin did this for his "prank"...our school has the same background wallpaper on all of the computers, which is loaded through the network. He took the Burger King logo (you know, have it your way?) and used Photoshop (maybe Illustrator, too) and put their class year as the meat (I think). The text said something like "Class of 2006...Have it our way", in whatever way it would have normally appeared in the logo.
Or something like that.
He talked to the guy who ran the network, and he let them use that as the network wallpaper. So, all of the computers had a BK/Class of 2006 wallpaper on them whenever anyone logged on to one.
No guarantees on how something like that would work for you, but it's pretty much harmless and shouldn't get you in trouble (assuming you can get the network guy(s) to work with you).