Do you know what the prank is that they're going to pull? Find the weak link and sabotage it so that they fall all over themselves when they try it. Don't do anything yourself that would be dangerous or harm them back.
For instance, if they plan to fill buckets with water and douse you, round up all the buckets first and sit on them yourself, preferably with some other girls, so that the older girls can't take them away from you without giving away their secret. Be sure to look wide-eyed innocent and keep asking "why do you need them?" when they come looking.