I decided to send Sarah Conrad to because I wanted her to have the kind of adventures and experiences that Nancy Drew did. It would be hard for Sarah to do that living at home, even with workaholic (and therefor absent) parents. For example, if Sarah wanted to go out at night, she would have to ask permission, explain where she was going and say why. At boarding school, Sarah makes these decisions by herself and for herself. It leaves a lot of room for character development and plot angles.
There's another reason why I sent Sarah to boarding school: I always wanted to go to one myself. From Madeleine to traditional British boarding school series such as Elinor M. Brent-Dyer's on to more mature modern works such as John Green's Looking for Alaska and E. Lockhart's The Disreputable History of Frankie Landau-Banks, I've been fascinated about the kind of independence that children and teenagers have at boarding schools.