Well Witched by Frances Hardinge

Ryan, Josh, and Chelle need money to catch the bus home from a place called Magwhite. They weren’t supposed to be in Magwhite at all, and if they can’t get home fast--they’ll be in big trouble. So the daredevil among them, Josh, jumps into an old well to gather up the change folks have tossed in while making wishes over the years. He gets enough to get them home and all seems well.

Then strange things begin to happen. Chelle begins to speak people’s thoughts out loud, Josh begins to cause electrical disturbances, and Ryan suddenly sprouts odd growths between his knuckles that look curiously like closed eyes. Through nightmares and visions they gradually learn that they must grant the wishes of the original owners of the coins. This is easier said than done, and chaos seems to follow their efforts—infuriating the witchy being at the bottom of the well.