No senior, but still a high school prank.
We called it Plan P. It took a bit of planning that all focused around one key item. It was an actual key. A master lock key for small Master locks. I've had it since elementary school. Found in on the playground and have found countless locks that it works.
The plan involved a mechanics shop with about 40 students. The class had a large TV mounted fairly high up on a wall. The TV was used to show demonstration videos before they went out to the shop to work on actual cars. The TV was hooked up to a cabinet further down the wall. This cabinet contained a DVD player which was locked... by a small Master lock.
The majority of my friends had this class at the end of the day, during a period when I did not have a class. My job was to cause a diversion (which I won't go into the details of). I gave my master key to another friend who was assigned to 'make the switch'. The third person involved simply needed to supply some material. I believe it was something like 'Anal Fuck Frenzy'.
This is how it all played out. Once the mechanics class went out to the shop, the Switch hid in a car with the key in possession. The diversion commenced which brought everyone out of class and the building. During this time, the Switcher unlocked the cabinet, placed the material, grabbed the remote for the TV, and the most important part, switched the lock for another one I gave him.
Afterward, he hid again. The class came back in and continued. Sometime after, they went back into class and things started to unfold. The TV turned on (to display an already playing DVD). I'm told the TV was on for a few minutes before the teacher realized what everyone was laughing at. This continued for about 40 minutes. Several faculty members came in to assist in removing the lock. Which eventually lead to them removing the hinged. By that time, the film had already reached a climactic point and several students had recorded the entire process.
It wasn't the craziest prank, but definitely a fun one.