The Ghost of Graylock (A Hauntings Novel) by Dan Poblocki

 

From Amazon:

Everyone's heard the stories about Graylock Hall.It was meant to be a place of healing - a hospital where children and teenagers with mental disorders would be cared for and perhaps even cured. But something went wrong. Several young patients died under mysterious circumstances. Eventually, the hospital was shut down, the building abandoned and left to rot deep in the woods.As the new kid in town, Neil Cady wants to see Graylock for himself. Especially since rumor has it that the building is haunted. He's got fresh batteries in his flashlight, a camera to document the adventure, and a new best friend watching his back.Neil might think he's prepared for what he'll find in the dark and decrepit asylum. But he's certainly not prepared for what follows him home.

My take:

I'm surprised that I liked this mystery made specifically for kids. Anyay, it turns out that the nurse never killed any of the kids .

Neil and his siter where sent to live with their aunts.Their mother was recuperating from a mental breakdown after being abandoned by her husband, their father.

Upom arrival at his aunts' place, Neil becomes obssessed by the Graylock Ghost legend of the neighboring abandoned asylum. Rumor has it that the abandoned asylum was haunted by the nurse who supposedly killed 3 patients.

After investigating the legend more thorougly, Neil realizes that the haunting was not made by the nurse but by one of the dead patients. 

The nurse was actually living in a senior home and was quite aware of the rumors surrounding her departure. After careful scrutiny they found out that the last patient who died demanded to be placed in the asylum after her mother died. He, along with his sister and 2 friends, realized she was escaping her step father who she suspected killed her mom. The murderer turned out to be one of their aunts' friends who always tried to help them anyway he can.

When they realized he was the killer who was about to murder them, the ghost of the girl helps them escape. The killer eventually dies.

After the end of the summer, Neil and his sister goes back to their mom who has fully recovered.