Three Towers Hall was a boarding school for girls For a short time after Billie arrived there all went well. But then the head of the school had to go on a long journey and she left the girls in charge of two teachers. sisters. who believed in severe discipline and in very. very plain food and little of it - and then there was a row! (From original advertisement) This is the second book in the "Billie Bradley" series, a mystery series for girls.