Dare them to do Bloody Mary but have someone hiding (Lookinglikebloodymary) in the shower, then after they do it have the candle or candles go out and then have "Bloody Mary" come out and scare the t out of them.
Ok, I did this funny prank at school once. Take a piece of paper and put it on a chair (paper should be same color as the chair.) Put clear glue on the paper and someone will sit on it and be walking around with paper on their but all day.