On a trip to the New York Aquarium with her third-grade class, a teacher discovers a dead body: "One of the world's shrewdest and most amusing detectives" (The New York Times).   For the third graders at Jefferson School, a field trip is always a treat. But one day at the New York Aquarium, they get much more excitement than they bargained for. A pickpocket sprints past, stolen purse in hand, and is making his way to the exit when their teacher, the prim Hildegarde Withers, knocks him down with her umbrella. By the time the police and the security guards finish arguing about what to do with Chicago Lew, he has escaped, and Miss Withers has found something far more interesting: a murdered stockbroker floating in the penguin tank.   With the help of Detective Oscar Piper, this no-nonsense spinster embarks on her first of many adventures. The mystery is baffling, the killer dangerous, but for a woman who can control a gaggle of noisy third graders, murder isn't frightening at all.   The Penguin Pool Murder is part of the Hildegarde Withers Mysteries series, which also includes Murder on the Blackboard and Murder on Wheels.