The Children of Wolf Rock by Natasha Farrant

High in the mountains of western Scotland, there is a loch, and on its shores there is a castle, and in that castle there is a school. Stormy Loch Academy is like no other teaching establishment. For one, its headmaster, Major Fortescue, believes in sharing the beauty of the world with his students, so that they will have the courage to fight for it. For Minna, Kass and Tom it proves to be the safe haven from their chaotic home lives, giving them all a chance to breathe freely, and to roam the valleys and lakes on their doorstep. But then they come across Addie, an older ex-student, hiding out in the wild, with only her guitar and her dog for company. At first they believe her stories, but when she goes missing, the lies she has told start to emerge. With Addie at risk, Minna, Kass and Tom must travel to London to uncover her past. The journey is dangerous, especially for those so young.