Orphan Monster Spy

After her mother is shot at a checkpoint, fifteen-year-old Sarah finds herself on the run from the Nazis in Third Reich-ruled Germany. While trying to escape, Sarah meets a mysterious man with an ambiguous accent, a suspiciously bare apartment, and a lockbox full of weapons. He's part of the secret resistance against the Reich, and he needs her help.
Sarah is to hide in plain sight at a boarding school for the daughters of top Nazi brass, posing as one of them. She must befriend the daughter of a key scientist to gain access to the blueprints for a bomb that could destroy the cities of Western Europe, and steal them.
Sarah may look like the rest of the girls, innocent, blonde-haired, and young, but she refuses to become one of the monsters she's surrounded by. She's a brilliant con artist, convincing them she's one of them even as she lives in terror of being found out. And she's determined to get her revenge on them all.