Let's see.... 1979 as I recall:
1. Seniors picked up an MG midget convertible, turned it sideways to get inside the door to the main hallway and "parked" it in front of the principal's office.
2. Hallway bell seemed to be way too loud... Somehow ended up in the Senior's time capsule which was opened in 1989... Wonder how that happened...
3. An extra diploma for a non-existent student was added in the stack of diplomas... The Dean almost read it aloud... Can't remember the name off-hand, but was quite a riot...
4. One of our buildings had exposed ceiling supports in the classrooms... Took about 10 minutes before our French teacher noticed two of us sitting up there like monkeys...
5. Watered down one of the walkways during winter, had about an inch of ice for over a week...
6. Staged a sit-in to be allowed to wear blue jeans... and succeeded...
7. Several Anarchist handbook experiments were performed in and out of the chemistry lab which I probably shouldn't advertise...

8. Not really a practical joke, but remember one overnight basketball trip where some of us "borrowed" the coaches keys, bought a case of beer, got pulled over, surrendered the beer and had to explain to the coach why we were innocent.... LOL
Now that I think about it, wish I could go back and revisit high school... I really enjoyed our "higher" learning!!!!
-whynot