Raising Arcadia by Simon Chesterman

Arcadia Greentree knows she isn't exactly normal. But then she discovers she isn't Arcadia Greentree either. Arcadia sees the world like no one else. Exceptionally observant, the sixteen-year-old is aware of her surroundings in a way that sometimes gets her into trouble and out of it again. But when she seeks to unravel a mystery at school, a tragedy at home forces her to use her skills to catch a killer.