These are a few pranks I've found so far
Alarm Clock Mayhem
Set alarm clocks to go off about 2 minutes from each other...hide them all around the school...in lockers, bathrooms, ceiling tiles, vents,
Tiny Bookmarks
Create thousands of tiny bookmarks saying something like Class of '0? - Senior Prank; or Class of '0? was Here. Put them in thousands of books in the library ... 10 years later people will still be finding them.
Letter Men
Everyone needs to get a t-shirt with a letter (or number) on it - you can make them with iron on decals. Just one big letter. Different colors, different styles, on the front or back. Throughout the day, stand near people making words in the hall...snap photos. So letters D, O, R, and K can run next to principal and quickly snap photo. They can't exactly make the whole class change, but they can however ask you put your shirt on inside out...so as a precaution, put another letter on the other side
Sleepover
The entire senior class camps out in the front yard of the school....and sleep in.
Parking Problems 1
Seniors, park your cars in a circle around the school...so close together that a person would have to climb over a car to get to the building.
State Flowers
In most states (check yours) it is illegal to dig up the state flower (or sometimes tree). So if you plant the state flower in the shape of your class year, then they cannot legally dig them up.
Pigs gone Loose
Get or borrow 3 pigs and label them 1, 2 and 4. Let them loose in the school and watch as everyone looks for the third pig!
Cement Year
Dig a hole in the grass the shape of your year...then fill with cement.2 points
For Sale By pwner
Put real estate ad in newspaper offering the school for sale.Parking Problems 2
Seniors, park your cars in a maze shape nice and early making the faculty navigate through.
Swap for Diploma
Have everyone slip something into the principal's hand when you shake hands at graduation. Examples: a marble, a bouncy ball, a cigarette (if he smokes), a penny, a fortune cookie fortune
Photoshop
Edit some photos of principal to be funny - include your class year...distribute in bulk.
Dixie Cups
Purchase 700-1000 paper dixie cups. Fill them up halfway with water. Flip them over and put them throughout all of the hallways. They will be hard to get off and once they're removed the water will come out!
Bubbly
Fill every toilet in the school with detergent or soap. Wait for someone to flush.
Delivery
Order items from take out several take-out restaurants to be delivered to you at school - prepaid with credit card.
Anyone Got a Snow Machine?
If not...rent one. Snowballs on the front lawn!
Letters to the Staff
Send letters to the faculty and staff (at home addresses - look them up). Have the return address label be something funny...that may have a spouse or kid scratching their head. Add letter inside stating that your class shall not be forgotten. Return Address Possibilities: National Association of Overprotective Parents, Parents Wanting No Drama (PWND), The New Kakistocracy (kakistocracy means government by the least qualified or most unprincipled citizens)
Doorknobs
Rub Vaseline over the doorknobs so that when the bell rings everyone's hands will be slimy for the rest of the day. If your hand rails are yellow like mine; get some mustard from the cafeteria and use that.
Post-it Note Mayhem
Come in early enough so this will work- If there is a certain hall in your school, get a whole bunch of post-it notes and cover that hall with them entirely. Write a message on a few so people will stop and look at them, even spell out something on the floor. Then, around school, have various post-it notes labeling: Class of 0?- Post-it Bandits
Class Relocated
On a nice day, move all chairs outside in class size groups. Post a room number on each group. Inside, on the door of each classroom post note like "Class relocated to the flagpole". Report to class..outside.
Something Smells Fishy
Take a lot of fish and other things that will smell bad after a day and place it all over the school- in lockers, vents, ceiling tiles, and toilet paper dispensers-a day ahead of time. Watch as the whole school is filled with the smell and people start to find your surprise!
Bouncing Halls
Buy thousands of bouncy balls-of all sizes- and distribute them to many seniors. During a planned time period-Like between the first two periods so they will be there all day-dump them all over the halls and stairwells.
Sign an assignment with the name of a controversial public official and watch as the teacher asks "who put their name as George. W Bush?)
Put about 1000 plastic forks all over each end zone of the football field the night before homecoming(face up, with the majority of the fork in the ground)
Put bird seed all over the teachers parking lot. The day after there was TONS of bird poop all over, and the day of it was infested with birds
Put tons of those pink plastic flamingos in the yard
For sale sign at the entrance of the school