The Dragon Prophecy by Dugald A. Steer

Brother and sister Dragonology students Daniel and Beatrice are studying when they discover that the evil Alexandra Gorynytchka is searching for the fearsome Hammer of the Dragons, a weapon that can kill hundreds of dragons in one stroke. They pursue her to the Lost Island of Dragons where she keeps her army of Frost Dragons. There they must follow the clues left by Beatrice Croke to find the Hammer before her. It is a really good book. It's a fun novel to read if you're interested in fantasy and adventure.