The Trouble at Wakeley Court by Clara Benson

When the Grand Duke of Morania learns of a plot to assassinate him, he sends his daughter Princess Irina to school in England, out of harm’s way. British Intelligence scent trouble and ask Angela Marchmont to investigate. But dark forces are at work, and when the Princess disappears in mysterious circumstances Angela must race against time to find her, before the throne falls and Morania is plunged into revolution—or war.