First off, this prank could get someone seriously killed. People are generally freaked out by hornets, yellowjackets and anything else yellow with stripes that buzzes. Some is irrational fear and some people are highly allergic to the venom from the stingers.

Take a vial of hornet attractant (you can find it for hornet traps in any garden supply store) and pour it onto a cloth of some sort. Leave that cloth in a place you want the hornets to show up. Any place with any opening to the outdoors will work.

When the hornets pick up the scent, they will come from every direction. You can end up with ten or hundreds.