Conspiracies (Shadow Grail, Bk. 2)

Spirit and her friends Burke, Loch, Muirin, and Addie have managed to defeat the evil force that has been killing students at Oakhurst Academy for the past forty years - or so they think. When a series of magical attacks disrupts the school, the headmaster, Doctor Ambrosius, calls upon alumnus and security expert Mark Rider to protect the campus - and to start training the students for war. Against whom - or what - the students don't know. As the only student without magic, Spirit doesn't trust Mark or his methods. She knows that Oakhurst isn't safe, and if she and her friends want to live long enough to graduate, they have to find out what is really going on - before it's too late.