Mage Confusion by Virginia G. McMorrow

Alex, a schoolmistress, is hounded by lifelong friends, Duke Jules and Queen Elena, to use her dormant magic that disappeared in a traumatic childhood event. Typical mages can control one element (air, water, earth, fire), but they suspect Alex is the rumored Crownmage who can control all and assist Elena to retain power. Meanwhile, Anders, a friend of Alex’s dead mother, helps a very reluctant Alex accept her magic. Although Alex isn't the Crownmage, she can change all four elements from one to the other. During her training, Alex discovers that her father is still alive and has answers to why Alex’s magic is so different—but Alex wants nothing to do with him. When Elena's mage council forces a confrontation, the only way Alex can wield power is by clutching her mother's pendant, which goes missing. Nevertheless, Alex will do her best to defeat the council’s mage. On that day, Alex must learn to trust herself. But can she?