You are an aspiring student, trained in the art of Prankjitsu with the goal of finishing his training and eventually become a master prank ninja. Complete areas by pranking citizens with low-level pranks with scary masks, whoopee-cushions, fake poo etc. and work your way up to the ultimate prank whilst not being noticed in order to avoid being caught by the Fun Police. Earn belts to proves your mastery and eventually pull the ultimate prank on the school's teacher and become the ultimate master of Prankjitsu. Careful of your classmates and their as they will also stop at nothing to become the master themselves.
Image credit: http://gamingsnack.com/mini-ninjas-pc/, http://guff.com/the-greatest-ways-to-prank-your-friends