San Francisco’s juvenile hall would close within three years under a proposal heading to the Board of Supervisors on Tuesday with a majority of elected officials on board, backed by prominent supporters.

Six of the 11 supervisors, the district attorney, public defender and other local officials have thrown their support behind the measure requiring the youth detention facility to close by the end of 2021. It would also create a working group to oversee the process and come up with rehabilitative and noninstitutional settings for youths who still require a secure placement.

The supervisors drafted the measure in response to a recent Chronicle report that documented a dramatic drop in serious youth crime that has left the state’s juvenile halls nearly empty and per-inmate costs skyrocketing.

The six supervisors, Shamann Walton, Hillary Ronen, Matt Haney, Gordon Mar, Aaron Peskin and Sandra Lee Fewer, are expected to roll out the measure at a rally on the steps of City Hall, backed by District Attorney George Gascón and school board President Stevon Cook as well as formerly incarcerated youths and other juvenile justice reform activists.

Public Defender Mano Raju has also endorsed the measure, as have Leif Dautch and Chesa Boudin, candidates running for the district attorney’s seat.

Mayor London Breed has proposed a more moderate approach to reforming San Francisco’s juvenile justice system. The bill to close juvenile hall would need the support of eight supervisors to override a mayoral veto.

Ronen, one of the proposal’s initial proponents, said jailing youths doesn’t work to reduce crime, and the rising cost of juvenile incarceration is unacceptable.

“The results are clear, we are doing more harm than good when we send young people to prison when they’ve committed a crime,” she said. “We just have to act, and we have to have the courage to act.”

The Chronicle’s “Vanishing Violence” stories found that the San Francisco Probation Department directed $11.9 million to juvenile hall last year, an amount that has remained relatively flat since 2011, even though the average daily population has been cut in half. As a result, the annual cost of locking up a youth has nearly doubled since 2011, reaching roughly $270,000 last year.

Similar trends have occurred across California, where almost every juvenile hall is operating at or below 50% capacity and the cost to incarcerate a youth has reached more than $500,000 annually in some places.

San Francisco’s juvenile hall is an outdated facility, based on an outdated model, that needs to be replaced by small, therapeutic settings serving 10 to 15 youths, Gascón said.

“It’s not just simply shutting down the facility, but creating the infrastructure to do the work effectively,” he said. “The key is we want to be thoughtful and effective and to remain nimble. That is good for public safety.”

If the legislation passes, San Francisco would become the only urban area in California without a juvenile hall — a historic and courageous move, said Patti Lee, who oversees juvenile cases in the San Francisco public defender’s office.

“Once again, I think San Francisco is going to be a leader,” said Lee. “All our juvenile attorneys are ecstatic — they never believed they would see this.”

The legislation would require the working group to review data on the needs of children placed in juvenile hall and determine alternative, community-based programs to serve them. In addition, the measure would create a Youth Justice Reinvestment Fund, and money saved by closing juvenile hall would be directed to treatment programs and alternatives to detention.

Meredith Desautels, a staff attorney at Youth Law Center who has been working closely with the supervisors to develop the legislation, said that about one-third of the young people detained in San Francisco’s juvenile hall last December had only misdemeanor charges, and half were awaiting release to court-ordered placements.

While a majority of supervisors had signed onto the measure even before it was introduced, it’s unclear whether Breed will back it.

In response to The Chronicle’s series, she proposed creating a blue-ribbon panel to reform the county’s juvenile justice system, addressing ways to reduce incarceration and racial inequities as well as how to best use the facilities, including juvenile hall, which is two-thirds empty.

On Monday, Breed announced the members of the panel, which will be co-chaired by San Francisco Human Rights Commission Executive Director Sheryl Davis and Corey Monroe, a 20-year veteran of the Omega Boys Club of San Francisco.

The group is tentatively scheduled to meet for the first time in mid-April and produce a report within six months.

“I am proud that we are able to bring together such a diverse group of leaders to provide their expertise, insight, and experiences on how to best reform our juvenile justice system,” Breed said in a statement. “While we have had success in greatly reducing the number of incarcerated youth in San Francisco, we need to take the next step and reimagine what our system will be in the future.”

As Breed’s panel initiates its analysis, the supervisors’ measure will move through the legislative process with a final vote likely to come in June. It’s unclear how much opposition the measure will face.

Chief Probation Officers of California President Stephanie James cautioned that counties like San Francisco need to be thoughtful when closing juvenile detention centers, which provide a broad range of research-based services.

“Otherwise,” she said, “we are just gambling with the lives of children.”

Ronen said she expects pushback from Juvenile Probation chief Allen Nance as well as the union workforce. Nance has already said he can’t imagine how the community or youths in detention would be safe without a juvenile hall.

“It seems like people feel that this is the right approach, and we’re going to have a lot of people and organizations who are standing with us on this,” said Supervisor Haney, adding there are concerns about what to do with youths who need a secure setting. “There are a lot of questions. But I haven’t heard a lot of outright opposition.”

Jill Tucker and Joaquin Palomino are San Francisco Chronicle staff writers. Email: jtucker@sfchronicle.com, jpalomino@sfchronicle.com Twitter: @jilltucker @joaquinpalomino