As some students from Nashville to North Carolina return to the classroom, National Education Association President Becky Pringle says the in-person learning standards set by the Centers for Disease Control and Prevention "still aren’t being met."

"Most schools, especially those attended by Black, brown, indigenous, and poor white students have severely outdated ventilation systems and no testing or tracing programs to speak of," Pringle said in a statement. "It’s time to fund proven mitigation strategies -- and it’s far past time for every governor to prioritize educator vaccinations."

In Chicago, 25,000 public school teachers will begin voting Tuesday night on a proposal for returning to classrooms, ABC Chicago station WLS reported.

ABC News’ Sophie Tatum contributed to this report.

Athletes will be tested for COVID-19 at least once every four days during their stay in Tokyo at the pandemic-delayed Summer Olympic and Paralympic Games, according to a "playbook" released by organizers Tuesday.

Over the past week, Olympics organizers have released different playbooks for each group of key stakeholders that outline COVID-19 protocols and rules of conduct for before, during and after the Tokyo Games, which are slated to open July 23. The playbook released Tuesday, which is aimed at athletes and team officials, warns that individuals could be kicked out of their events if they break protocols.

In addition to regular testing at the Games, athletes and team officials must take a COVID-19 test approved by the Japanese government within 72 hours of the departure time of their flight to the country, show proof of that negative test upon arrival and be prepared to take another test at the airport. Athletes will be barred from competing at the Games if they test positive for COVID-19.

Athletes and team officials will only be permitted to leave the Olympic and Paralympic Village, or other designated accommodation, to carry out the activities detailed in their "14-day activity plan," according to the playbook. They are not allowed to visit gyms, tourist areas, shops, restaurants and bars, among other places.

The playbook also asks athletes and team officials to keep two meters away from others and avoid "unnecessary forms of physical contact such as hugs, high-fives and handshakes."

The 2020 Summer Olympics were supposed to kick off in the Japanese capital last year on July 24. But in late March, amid mounting calls to delay or cancel the upcoming Games, the International Olympic Committee and Japanese organizers announced that the event would be held a year later due to the pandemic. They have been outwardly staunch in their determination to go forward with the Games ever since, even as Japan -- and much of the world -- face a resurgence of COVD-19 infections.

Clinical trials testing a combination of Russia's flagship COVID-19 vaccine, Sputnik V, with another developed jointly by England's University of Oxford and British-Swedish pharmaceutical giant AstraZeneca will begin in Azerbaijan later this month.

The Russian Direct Investment Fund (RDIF), which funded the production of Sputnik V and is responsible for its worldwide marketing, announced Tuesday that Azerbaijan's health ministry has issued a permit to conduct the trials, which will commence before the end of February.

"The research will be carried out over the course of six months in several countries with 100 volunteers recruited in each," RDIF said in a statement.

ABC News' Alina Lobzina contributed to this report.

An international team of World Health Organization experts investigating the origins of the coronavirus pandemic said Tuesday it's "extremely unlikely" that the virus was leaked from a laboratory in the Chinese city of Wuhan.

The Wuhan Institute of Virology, which is affiliated with the government-run Chinese Academy of Sciences, has collected extensive virus samples, sparking speculations that it may have caused the original outbreak by leaking the novel coronavirus into the surrounding community. The institute has strongly rejected that possibility.

"The findings suggest that the laboratory incident hypothesis is extremely unlikely to explain the introduction of the virus into the human population," WHO food safety and animal diseases expert Peter Ben Embarek said at a joint press conference with Chinese counterparts on Tuesday. "Therefore, [it] is not in the hypotheses that we will suggest for future studies."

The WHO team, which includes experts from 10 countries, is considering several possible scenarios for how the disease was transmitted to humans, leading to a pandemic that has claimed the lives of more than 2.3 million people worldwide. Embarek said it's more likely that the virus jumped to humans from an animal.

"Our initial findings suggest that the introduction through an intermediary host species is the most likely pathway and one that will require more studies and more specific targeted research," he said.

Transmission through the trade of frozen products was also likely, Embarek added.

As part of their investigation, the WHO team has visited key locations in Wuhan, where the first cases of COVID-19 were detected in December 2019.

A cluster of initial cases has been linked to a now-closed wet market in Wuhan. But Liang Wannian, the lead Chinese envoy who is working on the probe, said the market may not be the first location of the outbreak since transmission was also happening in other areas of the city at the time.

A review of mortality data, antibody tests of blood in blood banks in Wuhan and genome sequences showed there was "no indication of the transmission of the Sars-Cov-2 in the population" prior to December 2019, according to Liang.

There was also no evidence of "large outbreaks" in Wuhan or elsewhere before December 2019, according to Embarek.

ABC News' Karson Yiu contributed to this report.