Children's Books, Reference 

Breaking promises. . .Jessica  and Elizabeth Wakefield's father teaches them a  secret language called Ithig. All their friends want  to know the secret too, but the twins have made a  pact with their father not to give it away.  Elizabeth, at least, intends to keep her promise. But  Jessica can't help herself--she teaches her best  friend, Lila Fowler, the secret language. That  means that soon the whole school will know  Ithig.Now the class decides to use the  language against the one teacher they don't like--Ms.  McDonald. They will speak only Ithig in class the  day the school supervisor comes to review her. At  first, Elizabeth is angry at Jessica for breaking  her promise about Ithig--but now Elizabeth  wonders: should she break her own promise and warn Ms.  McDonald?DON'T MISS ANY OF THE  BOOKS IN THIS FABULOUS SERIES! - from Amzon