I am writing a book about a girl who is a spy?

Don't make her mom the headmistress, make her the headmistress' niece or granddaughter. The mission could be her final test to pass the school and turn into a bigger mission than anyone every thought like uncovering weapons trafficking across the canadian border (I say canadian because they always do Mexican). The school could be called something simple and acting as a front like "The All Girls School of Excellence" Some classes could be undercover101, gadgets210, and so on. Also back on the mom part, you could make it where her mom doesn't know and her grandmother or aunt was the one who recruited her. Maybe her mom wasn't recruited because she didn't have the right skills so she was left out.