A headmistress is the female principal of a school, particularly a private school. You might need to ask the headmistress of your high school for a college recommendation.
Private schools and prep schools often call the person in charge a headmaste if he's a man and a headmistress if she's a woman. It's more common to have a principal in the United States, especially in public schools. The word headmistress is commonly thought of as British, but it's increasingly likely that a British student will use the term head teacher instead, since it can refer to either a man or a woman.