Galloglass (Worldquake, Bk.3) by Thomas, Scarlett

Effie Truelove and her school friends Lexy, Wolf, Maximilian, and Raven must put their magical skills to the test. The Diberi, a corrupt organization intent on destroying the world, has returned and has something sinister planned at Midwinter.

But during a visit to the Otherworld, Effie is mistaken and imprisoned for being a galloglass—a dangerous, selfish islander. Meanwhile, Lexy is threatened by the vile professor Jupiter Peacock and Wolf embarks on a perilous journey to find his missing sister.

And back at school, Neptune the cat is bored. He’s used to lording over the other stray cats, but they’ve all mysteriously vanished. Where could they be—and how will he find them?

Can Effie and her friends reunite before their universe ceases to exist?