I Know What You Did Last Wednesday by Anthony Horowitz
It's dangerous being the younger brother of the world's worst private detective, but Nick Diamond's survived...so far. When the pair receive an invitation to a school reunion on a remote Scottish island, things seem to be looking up. But Nick's got a bad feeling, and it's not indigestion. And when he meets their fellow guests, the feeling only gets worse - especially when they start dying in ever more bizarre ways!