This is book 24 of The Tania Series. Tania and Tanisha come across an old spell that brings bad luck. They decide to use it on a boy who often teases them. They think it is harmless fun, because we all know spells don't actually work. But then terrible things happen to the poor boy. Tania feels really guilty. How did the spell work? Read on to find out.
Check out a detailed review of this book, done by Plusminus'N'More here.
Paperback: The story is available as a part of the anthology called Tania Is Wicked.