The war between Thomas White IV and David Baez began, like so many petty adolescent squabbles, over a girl. In 2003 Baez, then a 15-year-old junior at Ridgeview High School in Orange Park, Fla., accused a female classmate of trashing his pals in a series of e-mails. She showed the exchanges to her friend White, and for two weeks the youths spat insults at each other in the school's halls. Tensions escalated until the final bell on Sept. 12 of that year. As Baez walked to his locker, White came up behind him. A friend shouted a warning, but Baez turned and White floored him with a sucker punch to the face. He didn't get up. "I didn't know anything was wrong," recalls White, 19. "I just thought he was knocked out."

White's single blow brought catastrophic consequences: Freakishly, the impact separated two vertebrae at the base of Baez's skull and caused hemorrhaging. For eight days he lay on life support as his parents, Edgar and Maria, and two sisters kept vigil. "We prayed and prayed, but he never woke up," says Maria. On Sept. 20 Baez—a fine student who planned to study architecture—was dead. "I felt his last heartbeat," says Maria, 46, who works for the Urban League. "I can still feel that like it was yesterday."

No one would have blamed Maria Baez if she had demanded the maximum penalty for her son's killer, who was initially charged as an adult with manslaughter and faced up to 15 years in prison. Instead, she displayed astonishing mercy. Noting White's age and the unusual nature of the crime, state prosecutors offered a unique deal: Pending approval by the Baez family, the teen would serve just three years in jail and 12 years probation if he spent his time in custody speaking to peers about the dangers of youth violence. Edgar, 49, a retired Naval lab technician, left the decision to his wife, who not only agreed but made a tearful plea with the judge for leniency. "I just listened to my heart," she says. "In prison, Thomas would just learn more about how to be a criminal. How does anything good come of that?"

Now White is an unlikely activist, traveling every few weeks from Clay County Jail to schools in northeast Florida to urge kids to talk out their problems. He also screens Death Blow, a 15-minute documentary that reenacts the fatal fight and features interviews with both families. "Let's not sugarcoat this. Thomas murdered David," says prosecutor Shauna Wright. "But it's no benefit to the Baezes or him if he spends more time behind bars."

School officials and students say the presentations have an impact. During a March visit to a Clay County school, White, in shackles, haltingly took questions from kids. Asked what the toughest part of jail is, he said simply, "I'm not allowed to hug my mom." After the speech, 12-year-old Ashley Coleman and friends vowed to do more to avoid conflict. "He says, she says—a little bit of drama and someone can be gone just like that," says the seventh grader. "People take their lives for granted."

That's something both the White and Baez families, who have met occasionally since the crime, deal with every day. Although their son had been in trouble for fighting before, Thomas White III, 45, a retired Naval petty officer, and wife Teresa, 38, say he'd cleaned up his act and was excelling in school before the incident. When the boys began their dispute, both mentioned it to their parents. "I gave David space and didn't intervene," says Edgar Baez. "That will haunt me forever."

To this day, Edgar questions whether White can truly be rehabilitated. "A tree that grows crooked will never be straight," he says. Maria is more optimistic, visiting her son's killer twice since his sentencing. "I thought she was going to be mad at me," White says of their first meeting. "But she wasn't." Instead, Baez calmly expressed her grief. "I told him I still feel angry about what happened," she recalls. "And I asked him how he was doing."

What she hopes is that at least one young person can ultimately be saved: "Maybe this happened so we can make a difference in Thomas's life," she says. "I just want to know he's learned his lesson." It seems he has. "A strong person is one who will walk away," White tells audiences. "All it takes is one punch to change your life forever."

Richard Jerome; Devan Stuart in Orange Park