The Dominion Key (Joshua Dread #3) by Lee Bacon

The third book in the series! The summer is coming to an end and Joshua—along with his friends Sophie, Milton, and Miranda—are about to begin seventh grade. But when a trip to the mall turns into an attack by nFinity and a couple of Phineas Vex's goons, it becomes clear that they're no longer safe in Sheepsdale. To ensure their safety, Joshua and his friends must enroll in Alabaster Academy, a school for Gyfted kids. Located on an isolated island where there are only two types of weather (rainy and very rainy), Alabaster is filled with every kind of superpowered student you can imagine—not to mention a whole new species of bully. But when Alabaster comes under attack, Joshua and his friends are forced to escape once again. Their only hope for survival is to find a mysterious key that will enable complete world domination for whoever possesses it. But what if Phineas Vex finds the key first?