Now I’ve done quite a few pranks over the years, and some I pulled at my high school are still ongoing, even years later.
The best pranks require a lot of background research; the more you know, the more epic the pranks will be. And bonus points will be awarded to original pranks. Case in point: some intercom systems in public areas are accessed by an internal phone number calling into a voicemail box. All you need to know is the internal phone number (and extension) and the intercom is yours.
Since I don’t want to spoil all the best pranks I’ve discovered (the best branks are discovered, not learned :D), I’ll just share some easy ones:
Pop off keyboard keys and rearrange them. On nearly all keyboards, this can be done without permanent damage. My personal favorite is to switch the M and N keys.
Replace all writing implements in your teacher’s desk with Crayons.
If you still use a chalkboard, put pieces of chalk in the eraser.
If you use a whiteboard, get mini dry erase markers and embed them into the eraser.
Have fun with MS Word autocorrect.
Get several hundred “Happy Birthday!” signs and balloons and stick them on random lockers. Bonus points if you use a mathematical sequence instead of random lockers, such as prime numbers or the Fibbernaci sequence.
Make a giant cardboard/wood/PVC UFO, and make it look like crashed into your school. The crash site has to be highly visible. The UFO has to be at least 12-25 feet across. Put a fog machine inside, and then get random students to come to school in alien halloween costumes (or as much of a costume as your dress code allows).
Stage an Improv Everywhere style musical or freeze event in your school cafeteria.
Get cheap 3D glasses (the ones with the red and blue lens). Disperse them out to as many students as possible, as quickly as possible, then have them all walk through the hallways between classes with their hands out saying things like “Whoa! That looks so real!”.
Teach at least thirty or so students to go skanking [dancing]. Someone pull a stereo out of a bookbag in a hallway, cafeteria, etc. and blast something like Goldfinger by Superman. Why skanking instead of something more mainstream like rap/pop? Because it’s not what everyone would expect from “spontaneous dancing”. The best pranks shatter someone’s preconceptions, or make them think outside of the “I eat the exact same lunch everyday” mindset.
And remember, always use your powers only for awesome. Because when all is said and done, if it’s still funny, then it was a successful prank. It’s even better if it can momentarily break people out of the same mediocrity of life. 
