Graevale by Lynette Noni

Used Paperback. 

In her third year at Akarnae Academy for the gifted, Alexandra Jennings knows now more than ever that there is danger looming over the school campus and beyond. To save her newfound home, she must embark on a journey with her friends to unite the races of Medora against the rising threat of rebel Prince Aven Dalmarta. From the humans of Tryllin, to the Jarnocks of the treetop city Maroo and the Flips of the Undersea Islands, Alex appeals to foreign leaders for help in saving the whole of Medora. Because if the prince follows through on his promise to purge the land of mortals, everyone 's days are numbered. Proving the most difficult to convince, however, are the Shadow Walkers, a mysterious and dark-cloaked people with grey skin who travel via shadows. They reside in Graevale, a monochromatic city that is both day and night, stormy and pleasant. In an effort to warn them of the wars to come, Alex undergoes a dangerous testing that risks not only her life, but also all of those in Medora. In the novel 's climax, armies collide in Graevale as Aven mounts his attack on Alex and the city. Alex learns confronting truths about life, love, trust and sacrifice.