Quarter Day

It starts with a boy on a beach. He's running away from home when Arian Malcolm meets him, and he did it by opening a magical Portal to escape. She is sent to the Duchy of Montfermo to find out what has happened to him - and what happens to the magically Talented who are born there. Montfermo has cut itself off from the outside world for seventy years, and is only now allowing foreigners within its borders - and what Arian discovers there threatens the safety of all the surrounding lands.
This is a sequel to Like Father, Like Daughter, describing Arian's first solo mission as a trainee Harper.