I read it around the age of 10, late 80s. I'm really hoping I'm not conflating two stories here, but the plot as I remember it is a girl staying in a house in or near woods. Befriends the ghost of a girl who died long ago.
The living girl discovers the truth about the other girl's death by either witnessing it or reliving it "as" the dead girl. She was murdered by her (step)mother and buried secretly in the woods. There was a disturbing scene where the (step)mother burns the girl's wrists somehow.
I also think there was an element of the plot that took place in an old Victorian school, possibly with additional ghost(s), but that might be where I'm mixing up two stories.
It sounds really distressing and not suitable for kids, but I definitely read it as a child! I remember being very saddened by it and it's stuck with me all these years, but I have no idea what the book was.
Does anyone recognise it?