Harper Rose and the Catacombs of Imperium by Molly Regulski

Harper Rose is a peculiar girl who lives in a boring town where nothing ever happens. The only place she feels she fits in is her world of books under her special tree. But when a small creature gifts her a crystal and gives her insights into her magical powers about who she truly is, her world turns upside down. When she lands, literally, at the Imperium Mystical Magic Academy, her strange adventure begins. Harper learns about flowers that cast magical spells and can send you into the pit of your worst nightmare or into the joy of your most pleasant dream, controlling her surroundings with her emotions, and how to ride a Pegasus. But as the young teen girl spews Latin in her sleep, she and her four best friends search to find the meaning behind the riddle it presents, and the villain that might be lurking within the castle walls.