The Guardians of the Athame by Claire Hastie
A secret school hidden in the deep forests of Northern Ireland is protected by an ancient powerful magick. But an evil presence lurks in the forest. A portal to another dark realm was cracked and sinister energy is leaking through, killing everything that surrounds it. A group of eleven special children of Blackhill Manor face a race against time to develop their magickal powers, and stop the destruction from spreading. They will need the power of an ancient dagger called the Athame. The crystals that power it are missing and they must be found. Can twelve-year-old Luna Green lead the new Guardians of the Athame to find the missing crystals in time to protect the world from the Shadow Walkers and the darkness that will be unleashed?