I attended a tiny high school where "The Senior Prank" was a much anticipated annual event. I can remember epic pranks such as:

Lining a particular teacher's classroom floor with a thick plastic tarp, putting all the desks back, and filling the room approximately a foot deep with lime Jell-O.

Parking an MG Midget in the library (this involved taking the school doors off their hinges to get it through).

Creating a twenty-foot tall ziggurat made of empty Budweiser longneck bottles atop the roof of the school (it took about a year and much coordination between party planners to accumulate enough bottles -- it was several hundred cases worth -- had to rent a U-Haul to get them to the school, and it took five people all night to get them on the roof. And another five people to build the thing. I'm told. Ahem.)

Chaining all the parents' and teachers' cars into the parking lot on graduation day, and immediately leaving for Florida with the only key.

And then there was the day of the science fair where I had a bunch of dry ice left over from my cloud chamber project so I broke it up in chunks and put it in all the water and tea urns and pitchers in the cafeteria about three minutes before lunch started -- it looked REALLY COOL but I did get suspended for that, strangely enough....

During our televised morning announcements, some friends of mine collected clothing (male and female) and hid behind the backdrop thing and slowly started 'stripping' as the announcements started (see: tossing a belt over, then a blouse, then a bra, etc etc). It was HILARIOUS because the announcers were in on it, but they were totally nonchalant. Even when we missed and nailed one of them with a pair of pants.

We also had someone dressed up as the Black Knight who stood at the bottom of the spiral staircase (the main one we used to go between the two floors) and harassed people during the day (There were several, I'm told, but I don't know how they were all coordinated.) And then some people came running by doing the coconut thing.


My school was a bit crazy and everyone thought these were all hilariously orchestrated. (I went to a fine arts/humanities concentrated school and if we were going to do something--we did it ALL THE WAY DAMNIT.)

As far as a senior prank our alleged rivals pulled on us (we didn't really give a shit about them one way or the other, but for some reason, they disliked us), they spray painted SENOIRS all over our school. Misspelled, just like that. Oops. Allegedly, someone left a bunch of those 'commonly misspelled words' lists all around their school the next week.

A couple of us sneaked onto campus* in the middle of the night, rounded up all the big trashcans and then made a big trashcan pyramid on the roof of the main administrative building. We filled some off the base trashcans with some water and duct-taped the rest, so it was kinda sturdy. It was kinda dumb, but it was fun to sneak around in the middle of the night and then run away when a security guard pulled into the parking lot.

*Like most AZ schools, it was an outdoor campus--that is, all the classrooms opened up to outdoor hallways

Our pranks were pretty tame - we had one traditional one (painting our class year in big numbers on the street leading to the football field) that the administration tolerated in exchange for not doing anything else. Honestly, by that point we SO did not care.

Graduation day? You guys saved pranks for one day a year? ;)

I can't remember who started it but sometime in fourth grade, the daily ritual of putting our chairs on the desks to make way for the cleaner to be able to clean the floors became a game of "who can balance the chairs at the very very very far edge of the desk and still make it stand there". For a few weeks we moved our chairs just a wee bit closer to the edge without it falling all winking at each other. I remember winking to one guy a lot (I think he started it, he was the "bad guy") when I showed him that my chair was barely even on the desk that afternoon.
One day when class started, our teacher began with giving us a great big scolding for "almost giving the cleaning lady a heart attack" because as she came in and closed the door, every single chair promptly fell off the desk with a loud thud and crash.
That end of day, everyone tried to do it again, and I recall doing a lot of waving and winks to make everyone push their chairs back onto the desks. Even the bad boy knew the joke was done and obeyed.

Speaking of pranks.

We built a VW around the flag pole, Put a gaint fiberglass whale on the roof of the school, put a car on the roof and spray painted the field.

Earlier in High School we found all of our desks outside.