My favorite practical joke was performed back in high school on the director of our Audio Visual Dept.

My favorite practical joke was performed back in high school on the director
of our Audio Visual Dept. Upon purchasing a brand new, expensive video
camera, he set it up in the AV lounge so he could watch us hoodlums
on a monitor in his office. While he wasn't looking we taped a clear
piece of plastic over the lens of the camera. Then the designated
provacateur made sure that he was watching while another went up to the
lens with a squeeze bottle of highly caustic liquid, and crusty, grime
laden rag, to "clean the lens". Fred jumped over his desk and knocked
over a couple of onlookers before realizing what was going on.

( All the standard things happened to this guy too! remove all the screws
>from his chair, disconnect ( or reverse ) key components of his phone,
placed packing material in his fan. )