>Since the statute of limitations is probably up on this one, the biggest practical joke I ever pulled was a year after I graduated high school - I telephoned a bunch of high school seniors about a week before they were normally going to receive the standard phone calls from local army recruiters. I can do pretty decent voices, so I created a phony southern accent for a ficticious "Seargeant McWilliams from the Army Reserves". A few kids could tell it was me, a few didn't know it was me but thought it sounded fishy....but more than half fell for it. I had seniors waiting in the principals office at a specific date/time for a meeting that was never to be.
Along the same lines, Milovan Đilas, the enfant terrible of the Central Committee (and later a dissident and political enemy #1 or thereabouts) pulled the best one. Somewhere in the late forties, while the system was still fighting the remnants of the fascist/nazi groups and the Stalin's machinery was already blocking the borders where it could, he sent a telegram to twelve top members of the CC: "everything is discovered. run.". Eleven out of twelve did.