I taught middle school for six years and have read a great number of books aimed at that age group. What we think is a thriller, may not be a thriller to a middle schooler. One book I think boys will especially enjoy as a fantasy/thriller is Tunnels by Roderick Gordan and Brian Williams. There are four other books in this series: Deeper, Freefall, Closer and Spiral. The main character is a young boy from a dysfunctional family with an eccentric father. He becomes trapped in a subterranean world fraught with danger and intrigue and surreal characters. Another series of books that cross a bit into horror is The Haunting of Derek Stone series by Tony Abbott. There are 5 books in the series The Haunting of Derek Stone, The Red House, The Ghost Road, Bayou Dogs, and City of the Dead. These books are set in New Orleans and there is a lot of action and suspense associated with them. The crossover comes in the fact that the pursuers are spirits and pure evil. As I check my notes and library I will send you other books that I think might fall under that category.