Amid mounting tensions about reopening schools, the Centers for Disease Control and Prevention planned to release long-awaited guidance Friday on what measures are needed to get children back into the classroom during the pandemic.

President Joe Biden requested updated the guidance when he took office, responding to complaints that the CDC’s school guidelines under the Trump administration were unclear and inconsistent. The White House said Thursday that the new guidance would help drive a national strategy to get students back to classrooms.

Biden faces increasing pressure to deliver on his promise to get the majority of schools back to in-person teaching by the end of his first 100 days in office. The White House said this week that schools would be considered opened as long as they teach in-person at least one day a week.

There’s wide agreement that learning in the classroom is more effective and that students can face isolation and learning setbacks at home. And a study released by the CDC in January found that, with mask wearing and other measures, it’s generally safe to teach classes in-person. But teachers unions in some areas say schools have failed to make buildings safe enough to return.

Many Republicans and some Democrats counter that there’s increasing evidence that it’s safe to reopen schools now, and in some communities, students have returned to in-person learning five days a week.

Biden has been caught between competing interests as he works to get students in the classroom without spurning the powerful teachers unions that helped get him elected. Critics say he has bowed to unions instead of taking more aggressive action on reopening.

Unlike former President Donald Trump, who pressured schools to open and blasted the CDC for issuing guidance that he said was impractical, Biden has kept his distance from the CDC as it works on recommendations. Even after the CDC’s director recently said that vaccinations are not a prerequisite for reopening, the White House declined to take a firm stance on the question.

Getting students back in the classroom is seen as a key to getting parents back to work. As part of Biden’s coronavirus relief package, he’s calling for $130 billion to help schools update buildings, buy protective gear and enact other recommended safety measures.

He’s also calling for routine virus testing in schools to identify infected students and teachers, and to contain the virus quickly.

House Democrats this week advanced the education portion of Biden’s $1.9 trillion relief plan despite opposition from Republicans, who wanted to limit funding to schools that are teaching in-person. Democrats countered that the funding is needed to make schools safe.

Existing CDC guidelines encourage a variety of measures to fight COVID-19 in schools, including the use of masks in certain settings, along with social distancing, hand washing and regular cleaning.

Some education leaders, however, say the guidance does not go far enough. On masks, for example, it says face coverings are recommended if students cannot be spaced 6 feet apart, but with social distancing, it says masks “may be considered.”

Under the Trump administration, the CDC also was accused of issuing inconsistent information to schools ranging from guidance on screening students to whether whether entire schools needed to be closed if a single case of COVID-19 was confirmed in the building.

In the early days of the U.S. epidemic, some health experts worried that schools might become cauldrons of coronavirus infection, with kids infecting each other and then spreading it to family members — as seems to be the case during cold and flu season.

Those concerns were stoked by reports of an explosive outbreak in May at a high school in Israel, shortly after schools in that country reopened after a lockdown.

But with the economy reeling after lockdowns of schools and businesses last spring, the Trump administration pushed hard for schools to reopen.

In July, Trump accused the CDC of “asking schools to do very impractical things” in order to reopen. Education Secretary Betsy DeVos incorrectly said research showed there is no danger “in any way” if kids are in school, and Vice President Mike Pence promised that the CDC would issue new guidance.

The CDC did post revised documents late that month that kept many of its earlier recommendations. But in response to reporters’ questions, CDC officials said that the decision on whether to send kids back to school really rested with parents. The agency also posted an introductory document — written by government officials outside the CDC — that stressed the potential risks of children not attending school.

In an interview with The Associated Press last month, one CDC official suggested that it was only in late 2020 that many CDC scientists felt there was strong evidence that schools could be re-opened safely, in a way that keeps transmission low.

“My lens is really looking at the data that has become available in the last four to six weeks that has really advanced our understanding of how best to prevent transmission in schools, and increased our confidence that we can safely reopen many schools,” said Margaret Honein, the leader of a CDC task force working with state and local health departments, in a late January interview.