As a senior prank this year, the seniors started throwing water balloons everywhere during lunch. They blocked off the freshmen section of the commons and just rained down balloons on them all.
Then when the water balloons were done like...6 streakers ran through, some wearing jock straps, some wearing socks. One of them dropped his sock.
._.
And last year, about this same time, some kids went into a classroom for a project and turned on a fog machine. This fog machine filled the room with fog/smoke and set off the alarm. This was an hour before school, where the seniors and all their families were standing in line to get into senior breakfast. Everyone had to evacuate into the rain.