The Magic Mirror of the Mermaid Queen — Delia Sherman

Neef, the official Changeling of Central Park, has survived a life-threatening quest, but that's nothing compared to her first experiences at Changeling school. At Miss Van Loon's, she meets her counterparts from all over Manhattan, learns the basics of diplomacy, and, of course, gets in trouble. This time Neef must recover the Magic Mirror, or else New York Harbor's Mermaid Queen will turn all of the city's fresh water to salt -- and everything will die.