JOHN MURRAY doesn't look old enough to be in jail. Only 16, he still has the soft features of youth. It is not hard to imagine him on a playground shooting baskets with friends, or swinging a book bag on the way home from school.
But his face belies his acts. A convicted carjacker and armed thief, Mr. Murray is currently serving a 10-month sentence in the downtown Duval County lockup here.
Not long ago, Murray would have been tried as a juvenile, and received no worse than a 21-day confinement in a youth-detention facility. Facing a tidal wave of serious juvenile crime, however, more and more jurisdictions across the country are treating teen offenders as adults.
Jacksonville is at the forefront of handing out adult-size sentences to teens -- and thus offers a case study of how well such an approach works.
City officials credit the crackdown with having a dramatic impact on youth-crime rates. But experts nationwide continue to debate whether getting tough with teens is effective -- or proper.
''They are not being too tough,'' says Murray ruefully, ''not with me.''
Violent crime by youngsters is one of the most difficult and important issues public authorities now grapple with in America. Once elementary school students stashed only lunches in their lockers. Today some store guns.
School entrances are routinely framed by metal detectors. It seems that every night in major cities, TV news carries a story of a carjacking by one youngster or a shooting by another.
In response, most states have acted to make juvenile offenders face more serious punishment. Many have lowered the ages at which youngsters can be tried as adults. In Illinois, North Carolina, and Mississippi, children as young as 13 can be tried as adults for any criminal offense. In Idaho, Indiana, Iowa, and a raft of other states, the cutoff age is 14.
Jacksonville's experiment, a mixture of toughness and rehab-ilitation, is unusual. For one thing, this sprawling city some 37 miles from the Florida-Georgia line has the prison space to actually lock kids up, instead of sentencing them as adults and then releasing them on parole. The sixth floor of the new Duval County jail is now reserved for teenage offenders.
''From what we've been told, nobody in the country has more juveniles in an adult jail than we do,'' says Jay Plotkin, Jacksonville deputy state attorney.
For another, authorities here aren't just sticking kids in a cell and forgetting about them. The jail's sixth floor is also home to a public school run by the Duval County School Board.
Youthful convicts must attend classes, get counseling, and take part in a mentoring program. If the youth gets out of jail and completes probation without getting into trouble, his or her adult conviction is erased.
Jacksonville ''has developed a program where they have the services that a good juvenile facility should provide,'' says Howard Snyder, a researcher at the National Center for Juvenile Justice in Pittsburgh.
It all began in July 1991, when Florida's Gov. Lawton Chiles (D) appointed Harry Shorstein as state attorney for Jacksonville. Juvenile crime in Mr. Shorstein's domain was rising sharply at the time; for the year, it went up 27 percent. At the same time, the county jail, newly built for adults, stood half empty.
As in much of the rest of the nation in the early 1990s, juvenile crime in Jacksonville seemed a low priority for prosecutors. ''They were ignoring juveniles and they were becoming prolific [criminals],'' says Shorstein.
State attorneys in Florida have the authority to try almost any juvenile who is 16 or 17 as an adult. State law also provides for more youthful offenders to face adult treatment if they are charged with serious crimes.
Using this authority, Shorstein began trying juvenile repeat offenders as adults. The county jail was empty enough to separate youthful convicts from older criminals. ''What we decided to do is to send a message that there will be consequences for crime instead of this revolving door where a child gets before a juvenile judge and nothing happens to him,'' Shorstein says.
Over the past two years, Duval County judges have sentenced 442 youths under 17 to the county jail, 79 to state prison. One 17-year-old is on death row.
Clarence Jarrell, 17, is awaiting trial for carjacking. He says he will likely spend eight months in jail, if convicted. In the past, ''I'd have gone to juvenile detention and got out in 21 days,'' he says.
But does more harsh treatment of juvenile offenders work? Or does it simply harden them into confirmed criminals at a young age? In Jacksonville the trend in juvenile crime is positive. The number of children police arrested last year for committing crimes dropped 30 percent. Florida as a whole, by way of contrast, saw the number of juvenile arrests rise 23 percent last year.
But, in general, some experts claim that locking up juveniles in prison or harsh prison-like institutions is counterproductive. It reinforces adolescent aggression, while often depriving juveniles of the counseling services and positive adult contact they need to improve before their habits become confirmed in adulthood.
Some studies show that 75 percent of the kids who spend time behind adult-like bars will get into trouble with the law again.
''Adolescents who are committing crimes are not calculating before they commit the crime. Transferring these kids to adult court will not reduce recidivism,'' says Mark Soler, president of the Youth Law Center in Washington.