at one of the schools near where i live they bought in three real chickens and just let them run around the school. They labeled them one, two and four so the whole day everyone in the school was looking for number three, but there wasnt one :P pretty funny
Brainwave! Put dry ice down the toilets or in the pool! XD
Steal the Head's door, and then lock it to a lamppost with a ransom note attached.
A long time ago my last day in school we screwed over our history teacher. Put laxatives in his drink, broke the door handle so he couldn't leave once in the room haha. He soiled himself. After that it was the old bog roll/water mix plastered everywhere. Then got smashed. Good times.
2000 plastic ducks in the duck pond. glued the sixth form centre doors shut. penis's EVERYWHERE. standard.
We emptied a classroom and rearranged it out on the sports surface. Stole the biology skeletons, dressed them up and left them in the car-park. Filled a couple of offices with balloons. Newspaperd the staff-room door. Stretched duck-tape across the stairwells so that people couldn't climb the stairs. Everyone with cars brought them in and parked in the staff car-park so they had to try and find someone in the surrounding area to park.
Not anything huge, but enough to raise some smiles.
No pranks with us - we actually wanted to sit our exams in the coming days.
But . . . 3 sheep, numbered 1, 2 and 4 ; )
Put porn on all the projectors/screens.
Someone set off smoke bombs on our field. Covering it with red smoke.
Vaseline on door knobs, water, silly string, cling film on toilet seats, laxatives in water, graffiti, stealing toilet paper. Oh, and one year at our school, sixth formers stole the chairs out of the staff room and piled them all up at the end of the school field. Was funny.
on the ceiling if you have the tiles that you can lift up and down, we put fish up there so then it stinks the whole place out, or tinned tuna down the radiators, smells great and really hard to clean!
Find you're way to the kitchen and put some laxatives in some of the food. Then put itching powder on all the toilet seats. [You will probably get in to trouble and it will be a good idea to bring a packed lunch.]
Some form of fish in the heating system. That smell was unbearable but highly amusing.
This one was quite amusing at my school. Somebody taped an old mobile phone to the bottom a chair in a whole school assembly. From a totally different seat in the hall, somebody phoned that old phone and for a good few minutes straight all you could hear was Eye of the Tiger! They couldn't find it because they didn't know where it was coming from.