First off, never do this prank if there is a danger of trapping someone in a room.

Take a can of expanding polystyrene foam and squirt it under a door that opens into a room. The foam will expand and harden creating a very difficult to dislodge doorstop.

If the door has weather stripping along the bottom, take a screwdriver and gently lift the skirt so you can squirt under it. The great thing about the weather stripping is that it keeps the foam contained on the far side making it harder to figure out why the door will not budge.

If you can get access to the other side of the door, use some clear plastic tape to seal the crack under the door to prevent it from seeping out and being noticed. While you are at it, seal the hinges and any cracks around the door.

The foam can be chipped away, but it is a very messy job.