Night and day / Robert B. Parker.

Enhanced descriptions from Syndetics:

Paradise, Massachusetts, police chief Jesse Stone confronts a townas darkest secrets in the shocking new novel from the New York Timesabestselling author and aAmericaas greatest mystery writera (The New York Sun).
Things are getting strange in Paradise, Massachusetts. Police Chief Jesse Stone is called to the junior high school when reports of lewd conduct by the schoolas principal, Betsy Ingersoll, filter into the station. Ingersoll claims she was protecting the propriety of her students when she inspected each girlas undergarments in the locker room. Jesse would like nothing more than to see Ingersoll punished, but her high-powered attorney husband stands in the way. At the same time, the women of Paradise are faced with a threat to their sense of security with the emergence of a tormented voyeur, dubbed aThe Night Hawk.a Initially, heas content to peer through windows, but as times goes on, he becomes more reckless, forcing his victims to strip at gunpoint, then photographing them at their most vulnerable. And according to the notes heas sending to Jesse, heas not satisfied to stop there. Itas up to Jesse to catch the Night Hawk, before itas too late.