Sell your school on eBay. I was gonna do it, but I've got a much better one I'm doing instead. I'm not really gonna go into it though, because I'd be screwed if they figured out that I pulled it.
One good one I've heard is faking a snow day (call news/radio stations. But really, some of the best senior pranks are almost inside jokes within the school. Some prank on a certain teacher or relating to some unique thing about your school. So while any of the ideas you get on here will probably be good pranks, an idea that you come up with that is unique to your school/area will probably be much more successful.