Post the pranks you've done in your school or any ideas you have. For my senior prank a few friends and I are deciding between a few. The first is taking 3 medium sized animals and putting the numbers 1,2, & 4 on them making the school look forever for the nonexistent #3. The second is taking a shitload of bouncy balls into a fairly large bucket and dumping them into each separate courtyard halfway through the school day. The last idea is to take 100 chickens and dump them in the school which we filled with chickenfeed ahead of time.
As said before, I filled a guys room halfway with green Jello while he was on vacation. He called me, invited me over, and I ate a bunch of it. This cost about $100 and 3 hours to set up.