When the ninth grade's nice student teacher asks three boys to befriend a Juvenile Hall regular who will soon be released to enter their class, she admits that the prospect of their being able to help him is ""improbable."" As Ms. Karnisian has warned them, Arnold ""steals, he lies, he cons everybody around him."" He acts rude and tough, but is transparently insecure. The class does turn out for Arnold, however, especially after he takes the rap for brainy, overweight Warren, who has disconnected the class PA system for relief from the principal's announcements. With aid and encouragement from the others, Warren coaches Arnold for a Constitution test he must pass to graduate from junior high; and when Arnold chickens out and stays home the day it's scheduled, they arrange for him to make it up. Arnold is all set, and all hyped up, for graduation and the traditional dance. But then Jennifer, a girl Arnold has liked from the start, has to tell him that her snobby father won't let her go to the graduation dance with him. Arnold skips out with Ms. Karnisian's gift money, steals a car, and ends up back at Juvenile Hall. Mike, one of Arnold's original friends, is knocked out--and further discouraged because Ms. Karnisian, a great teacher though kids say she looks like King Kong, hasn't landed a regular teaching job for September. This hard-knocks ending isn't what you'd expect from the rosy improbability of the kids' efforts and progress with Arnold; but it's not an unprepared jolt. (Nor are Arnold's or Ms. K's fates sealed forever.) The realism and the good-natured optimism blend comfortably, the characters (especially Warren) have some color, and the liberal dabs of junior high school atmosphere make the whole encounter easy to visualize.