Shadow Island (Sueño Bay Adventures #1) by Mike Deas & Nancy Deas

From Orca Book Publishers: 

With nowhere left to live after losing his parents and working his way through a long list of relatives on the mainland, Ollie finds himself at his grandpa's house in Sueño Bay, "home of the supernatural."

Miserable and intending to get away from the earthquake-prone, rain-soaked island on the next bus out of town, Ollie's getaway plans are altered abruptly when he and three classmates discover something they shouldn't have in a trailer in the woods.

The four unlikely friends come together in a mystery involving supernatural creatures, a ticking clock and one angry gym teacher to save themselves, the creatures and Sueño Bay from a darkness that is infecting the island. If they succeed, will Ollie finally find a place to call home?