There are various myths about piranhas like they can dilacerate a human body or cattle in seconds. Those myths refer specifically on Pygocentrus nattereri, the red piranha. A recurrent myth is that they can be attracted by blood and are exclusive carnivores. In Brazil, there was a myth created called "Piranha Cattle" saying that they swept the rivers at high speed and attacked the first bovine as it entereed the water. It does not seem like it would be to difficult to replicate this myth with properly stimulated piranhas and a equivalent amount of beef to the average human male versus an average sized cow. All the best.

Piranhas are actually rather shy and reclusive by nature and will only attack something the size of a human if said human is already dead. To my knowledge there has never been a confirmed attack by piranha on a living human ever. Wikipedia artical on Piranhas

I seem to recall a nature show on TV that had footage of a rat-sized rodent being attacked by pirana. It was killed while swimming, but definately NOT skeletonised, or human sized. I remember also seeing footage of Brazillians swimming with piranha and not being bothered.