This is an event that you can use to involve everyone in the whole school in an amazingly short amount of time. You can do it as part of announcements and have your principal judge the finalists in the Main Office (start in homeroom and send each winner down to the front foyer or hallway outside the Main Office. This can also be done at a Pep Rally to involve all the students in one room.
The winners get the Rock Paper Scissors trophy to have and to hold as the champion of the school until the next contest.
You can learn more about the intricate skills of this highly technical game at the World Rock Paper Scissors Association website here.