Alchemy of Souls: The Hundred Halls Series Book Three

In the Hundred Halls, students experiment with all manners of magic: catoptromancy, lexology, verumancy, rune spells, demonology, shape changing, and a thousand other arcane techniques. Every magic is encouraged. Every magic, except one-soul magic. As Aurie's third year begins, students are found without their souls, ripped from their bodies, leaving them empty husks. Someone, or something, is stealing them for a nefarious purpose. With that kind of power being collected, not even the professors are safe. If Aurie and her sister cannot stop the soul thief before the end of the year, the Hundred Halls will never be the same.