Which faculty member at an old-fashioned boys' prep school is a secret psycho, a homicidal menace to teen-age lads? That's the not-very-original premise in this ploddish first novel, which pads out the thin whodunit with soap-opera subplots (primarily of the adolescent variety). Up in New York City, over Thanksgiving weekend, a young male prostitute is murdered (a neat neck-snap) on 42nd St. And the only clue (left near the body) is a receipt from the school store down at the Montpelier School for Boys in Virginia. So suspicion falls on the several Montpelier teachers who were vacationing in N.Y.C. or otherwise un-alibi-ed--especially when two of the schoolboys also turn up dead. Is the killer poetry prof Ben Warden, who's insecure because of a facial birthmark? Or drama coach Daniel Farnham, who has a crush on Warden's beautiful (but seriously ill) wife? Or. . .? The identity of the lunatic-among-us comes as no real surprise; his psychosexual motivation, as set forth in a corny confessional monologue, is thuddingly implausible. And the novelistic filler--various boys' growing pains with class bullies, shy girlfriends, secret tunnels, etc.--remains trite and humorless. In all: a hard. working, earnest, but flatfooted debut, only for those irresistibly drawn to the prep-school milieu.