I really doubt the police are going to do anything. They don't have the time (or the rights) to like individually search hundreds of kids because one of them lost his iPod.
Another reason why there's no use is because the school day is over. It's at the kid's house probably and, once again, the police aren't going to receive search warrants for every single kid's house to look for an iPod.
I'm pretty sure you stated this but the schools usually have the whole "If you bring your electronic devices to school and lose it, it's your fault" rule.
It wasn't smart to set it down and I cannot even think of any person at my school who would do that. At least you should have put it IN your locker or even IN a pocked of your backpack while you simply took off your shirt or something. Carelessness If I do say so myself.