Witch Week
Diana Wynne Jones
 This book is set in an alternate modern-day England (World Twelve C), identical to our world except for the presence of witchcraft. Despite witches being common, witchcraft is illegal and punishable by death, policed by a modern-day Inquisition. At Larwood House, an underfunded boarding school that many of the adolescent children of executed witches are sent to, a note claiming "Someone in this class is a witch" is found by one of the teachers. This launches an internal investigation of several of the more unpopular students at the school, some of whom are gradually coming to terms with the fact that they can do magic. In the traditional manner of children, magic and mischief, mayhem gradually ensues as magic is used to make birds appear in the classroom, to rain shoes, to curse a classmate into having his words always be true, and to do the traditional flying on a broomstick. When the magic gets totally out of control, one of the students runs away, blaming the witch for controlling him. This launches an investigation and the Inquisition is called to locate any witches and have them burned. Four of the students escape the school, two of them turning for help to an old part of an underground railroad system for witches to send them to another world where they'll be safe. While the old woman who lives there tells them the system broke down long ago, she does give them a spell to say at the Oak Grove that will summon help in an emergency. The four students and Brian, the runaway, gather at the Grove and say "Chrestomanci" three times, which summons the nine-lives enchanter from The Lives of Christopher Chant to help them. With his help, and the help of their classmates, most of which are witches themselves, the kids outwit the Inquisitor and ultimately revise their world's history by merging their world (Twelve A) with ours (Twelve B), which has no magic. When the note is found in this world, everyone exclaims they are the witch, and it is seen as normal.