Half of all U.S. 12- to 17-year-olds have had their first vaccine shot, White House COVID-19 coordinator Jeff Zients said at Friday's briefing, which he called "critical progress."

"The vaccination rate among adolescents is growing faster than any other age group," Zients added.

CDC Director Rochelle Walensky said a new CDC report out of Los Angeles County showed that schools following the health agency's guidelines had 3.5 times lower cases during the winter peak as compared to case rates in the community. (This study did not account for the delta variant).

"Even when communities were experiencing high levels of COVID transmission, in the LA County study, layered prevention measures in schools provided a shield of protection, helped to keep COVID out of school and reduced the spread when cases did occur," Walensky said.

She continued: "I want to strongly appeal to those districts who have not implemented prevention strategies and encourage them to do the right thing to protect the children under their care."

-ABC News' Cheyenne Haslett

A Tallahassee judge ruled Friday that Florida school boards can enact student mask mandates, going against Gov. Ron DeSantis, who in July issued an executive order that set off a public feud with school districts.

Judge John C. Cooper found that a blanket ban on face masks in schools "does not meet constitutional muster" and ordered the Florida Department of Education to stop enforcing a state rule that requires districts to allow parents to opt-out of mask mandates.

Cooper, over about two hours, picked apart the state's defense, saying the science presented during the weeklong trial did not support the state's argument against masking children.

Ten Florida school districts have adopted mask mandates for students this month. Two -- Broward and Alachua -- were sanctioned by the Florida commissioner of education, who said he would begin withholding the salaries of board members who voted to require masks.

DeSantis will "immediately appeal" the ruling, the governor's press secretary, Christina Pushaw, said in a statement. "This ruling was made with incoherent justifications, not based in science and facts," Pushaw said.

A spokesman for the Florida Department of Education said, "We are immensely disappointed that the ruling issued today by the Second Judicial Circuit discards the rule of law. This decision conflicts with basic and established rights of parents to make private health care and education decisions for children. ... We are committed to the fundamental rights of parents and will push forward on appeal to ensure that this foundation of democracy is upheld."

-ABC News' Will McDuffie

As Kentucky Gov. Andy Beshear announced the state's third-highest day of COVID-19 cases, he said at a news conference Thursday, "I’m gonna admit up front today I’m a little emotional and a little raw."

"Seeing nearly 5,000 cases and 65 Kentuckians that we lost in just one day’s report is tough," the governor said, adding that "it is entirely preventable."

Hospitalizations have increased every day in the last 42 days, he said.

On July 14, there were 60 people in intensive care units. As of Wednesday there were 549, he said.

At the beginning of the summer Kentucky's positivity rate fell to 1.79%. It's now soared to 13.16%, he said.

COVID-19 vaccinations in Houston increased more than five-fold on Thursday as the city launched a new incentive program.

The Houston Health Department is now providing up to $150 in gift cards to get vaccinated against COVID-19. A total of 740 vaccine doses were administered at the health department's eligible sites on Thursday, the first day of the program, marking a 51% increase over Wednesday's total of 121 doses.

Of the total shots administered Thursday, 658 were first doses and 82 were second dose, according to a press release from the health department.