THE "mass processing" of juvenile offenders through training schools is an old model that has begun to yield to a new vision in the corrections field - rehabilitation within the community. "In a sense it's kind of structured, benign baby-sitting to get kids through a tough period," says Barry Krisberg, president of the National Council on Crime and Delinquency, a San Francisco-based research and policy group.This community-based approach, pioneered by Massachusetts in the 1970s and increasingly copied today, takes a variety of forms - specialized group homes, day treatment, family-based services, and intense one-on-one supervision. Associated Marine Institutes, a Tampa, Fla.-based organization that operates in eight states, literally takes delinquents out to sea, using deep-sea diving, marine biology, and boating as vocational-training "hooks." Another respected, multistate option to incarceration is the outreach and tracking program conducted by the Key Program Alternatives for Youth, a nonprofit human-service agency based in Framingham, Mass. Juvenile offenders live at home, but case workers communicate with them an average of three times a day and keep tabs on their activities around the clock, holding them to the terms of a personal behavioral contract. Those who break the rules may be reassigned to group homes, a more restrictive environment but one that is still far less costly ($20,000 per youth annually compared with $50,000, Mr. Krisberg says). Such homes are often more humane than large facilities, which he says can seem like "junior penitentiaries. Rather than socializing the youth into community life, they socialize the youth into prison life."