The Denehurst Secret Service by Gwendoline Courtney

A school story that contains a mystery and is also an evacuation story – who could ask for anything more, as my friend Fred Astaire would say . . .

Mrs. Sherbourne has news for her two teenage daughters, Elaine and Moira. She is sending them to Denehurst, a boarding school in Cornwall, because London in war time is dangerous and their father needs to concentrate on his Foreign Office work. The girls are mildly indignant because they want to do their bit but cheer up when their cousin, Captain Deryk Holroyd, says he might need their help from school. He asks them to conceal their fluency in German, where the family spent three years.