The Winter Frost (School for Unusual Magic #2)

"After a chaotic semester of Elementary Magic, Rose, Amethyst, and Lav have found themselves with new magical skills up their sleeves which will come in handy when they begin Middle Magic next year. But school is the last thing on their minds over winter break. Tensions are high between the Land Walkers and the Merfolk's mysterious leader Frost. Worse, Amethyst's mom has gone missing with an extremely illegal wand, and everyone--their classmates, Principal Ivy, the Committee--thinks she's somehow involved in the disappearance of Rose's cousin Heather. But Amethyst is determined to find her mother and set everything right"--