Since senior pranks always tend to be fun stories, lets have a senior prank thread.
I'll start:
http://www.youtube.com/watch?v=wKGOBwX_vDI
The video explains it much better than my words could. We spent about 6 months planning and 'fundraising' for the prank. As a side note, the school was not damaged in any way shape or form and we bought the car ourselves.