From where I stand - Tabitha Suzuma

Raven is a deeply disturbed teenager, who, after witnessing the death of his mother, is placed in foster care. The Russells do their best to earn his trust, but only little Ella manages to get through to him.
Meanwhile, at school, bullies are making his life a living hell. An unexpected companion comes in the form of Lotte, a classmate bored by her 'ordinary' friends. Together, they track down Raven's mum's killer, with the goal of exposing him to the police. But their carefully crafted plan goes dangerously wrong and suddenly nothing is as it seems. Everything is falling apart and, ultimately, there is only one, final way out.