Nightschool: The Weirn Books, Volume 2 (Nightschool: The Weirn Books #7-12)

When Alex's sister, Sarah, vanishes and all memory and evidence of her existence is erased, Alex is determined to get to the bottom of her sister's disappearance. What better place to start her investigations than the Nightschool itself? But when she discovers that sneaking into the Nightschool isn't as simple as it might seem, Alex enrolls as a student. But is she prepared for what she might find?