The modern school equivalent of cheap pranks on computers isn't some elaborate virus, it's just pressing the "mail" or "calculator" keys on the keyboard for the guy next to you. Never personally witnessed anything more elaborate, though my classmate apparently distributed dubious batch files he wrote once
How about the one that launched a dialog box: "Do you have a small penis? Yes/No", and if you moved your mouse near the "No" button, the button would run away around the screen?