Author: Diane Wynne Jones
Witch Week is either the third or fourth book of Jones's The Chronicles of Chrestomanci series, depending whom you ask. All of the books are connected through a single character, Chrestomanci, more a title than a character actually - one assumed by various people throughout history. Otherwise, each novel is stand-alone so the order really doesn't matter. Jones suggested one order, various publishers a different one. It is my fantasy-loving daughter's favorite book series and generally a satisfying one for me, too.
Witch Week takes place at a fictional boarding school in a world where, unlike other dimensions we visit throughout the series, magic is outlawed. The story's two main characters, Nan and Charles, discover separately that they are, in fact, witches with considerable powers. They must hide the fact as witches, once revealed, face arrest and burning at the stake. Alas, neither can resist testing their newly found talents.
J. K. Rowling had to have read this book before creating Hogwarts. Influence from earlier fantasy writers is inevitable, of course. This is the first time I've seen obvious links with Jones's work: boarding school setting, adolescent social struggles, tensions between the magical and the mudane. Charles, like Harry Potter, wears distinctive glasses. Like Harry, Charles and Nan are both orphans. There's even some of the goofiness that Rowling incorporates: at one point, characters can't find brooms to ride so they must settle for a mop and a hoe. From what I've seen of her writing so far, Jones never quite matches Rowling's world-building but her characters are well fleshed out and her language a touch more sophisticated.