The Hunt for the Seventh by Christine Morton-Shaw

Jim moves to ancient Minerva Hall and encounters the ghosts of six children. They urge him to find the seventh child and leave him cryptic clues that point to a dark, ancient prophecy that only Jim can stop from being fulfilled. Jim turns to Einstein, a brilliant autistic boy who lives at the Hall. If anyone can help Jim, Einstein can. But the boy, who speaks in riddles, proves to be as mysterious as the dead children. Time is running out; if Jim doesn't figure out the clues, innocent people will die.

Christine Morton-Shaw has linked ancient rites with modern mystery to create a chilling, suspenseful tale that will keep readers guessing to the very end.