All of these pranks require access to the school for a solid chunk of time during which the school is unoccupied.
"Styrofoam Net": Buy some plastic mesh sheeting from a home supply store, the kind you can use to cheaply animal-proof a garden. Make sure it is see-through. Then, find a large quantity of styrofoam packing peanuts, like 10 cubic feet worth, which shouldn't be hard to obtain from a warehouse or other place that does shipping. Then, on the ceiling of a well-traveled hallway, non-destructively attach a surface of the mesh large enough to contain half of the peanuts, making sure to load them all in before you fully attach the surface. Now, do the same thing again right under the first surface, except this time permanently attach the lower surface to the top surface IN THE CENTER REGION OF THE SURFACE. If you wish to include a message of some sort, prepare it on a large sheet of paper, easily visible from the floor, and place the paper face-down inside the second surface. Finally, load the remaining packing peanuts into the second surface, above the optional message, and seal that surface as well. The idea here is that the school officials are faced with a choice: leave your message on display to anyone who walks down the hallway, or disassemble the aparatus and deal with cleaning up all the packing peanuts from the floor.
Kool Aid Cups: (This idea came from a friend) Obtain a large quantity of dixie cups, the smaller the cups the better. In a well-travelled hallway, line them up in rows with a density that makes walking through them impossible. The more rows the better, 10 feet worth seems ideal. Yes, the lining up process will be tedious, but not impossible. Afterwards you fill each cup with an inconvenient (but not permamently staining!) liquid. Obviously this will requires a container that will hold a large volume of the liquid and a hose to precision-fill the individual cups (no spilling). Congratulations, you have just accomplished the prank equivalent of violating the conservation of energy law from the perspective of human labor by putting more potential energy into a system than the work you expended on it.
The "get something really big into the school that they can't get out" Prank: The diversity of pranks following from this single concept is endless as long as you follow the simple principle assembly. Obviously, the quicker the assembly time and more extensive the object the better. If you are skilled with carpentry, you might build something out of wood, for the purposes of benign obstruction or just visibility. A particularly clever idea, I think, is to use water from a water supply to inflate a giant inflatable pool in the middle of some particularly inopportune location. Then, fill the pool with water as well.
(Note: All of these pranks cause incovenience. However, none of them are ultimately capable of causing any damage. If your goal is to cause damage, you don't need to try to be clever about it. Don't bother looking to these tips for help.)