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The president of America’s second-largest teachers union is calling on all schools to be open for "five days a week" of in-person learning for the next academic year. 

Randi Weingarten, the head of the American Federation of Teachers, made the declaration Thursday as about only half of the nation’s school districts are back for fully in-person learning, according to data from the Return to Learn Tracker website. As of early May, Florida and Iowa are the only states in the country that have all of their schoolchildren back in classrooms, while every other state is still offering varying degrees of in-person and online instruction, the data shows. 

"Conditions have changed. We can and we must reopen schools in the fall for in-person teaching, learning and support. And we must keep them open fully and safely five days a week," Weingarten said in a speech.

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"Given current circumstances, nothing should stand in the way of fully reopening our public schools this fall and keeping them open. Of course, it's not risk-free," she added. "We can manage the threat by encouraging people to get vaccines and following guidance from the CDC that prevents the spread of the disease."

Teacher unions in some states have complicated the school reopening process as they have demanded coronavirus vaccinations and other mitigation measures before returning to classrooms. 

But Weingarten argued that "critics have scapegoated teachers and vilified unions because of the school closures during the pandemic, ignoring the extreme disparities among schools and blaming teachers for problems outside their control.

"Creating safe conditions in schools during a public health crisis is not an obstacle to reopening to classrooms -- it's the pathway to going back, staying back and creating trust through the school community," she said.

Weingarten also said in her speech that "prolonged isolation is harmful" to students. 

White Plains High School students walk between classes on April 22 in White Plains, N.Y. (AP) (AP Photo/Mark Lennihan)

"School is where children learn. It’s where they work together and play together," she said. "It’s where they form relationships and learn resilience. It’s where many children who otherwise might go hungry eat breakfast and lunch." 

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The head of the union said 89% of its 1.7 million members are fully vaccinated or want to be.

"The United States will not be fully back until we are fully back in school. And my union is all in," she said.

Meanwhile, Weingarten’s union is under fire as top Republicans on the House Judiciary and Oversight committees have called for an investigation into the Centers for Disease Control and Prevention after emails showed the American Federation of Teachers lobbied the agency on school reopening policies. 

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House Judiciary Committee Ranking Member Jim Jordan, R-Ohio, House Oversight Committee Ranking Member James Comer, R-Ky., and House Education and Labor Committee ranking member Virginia Foxx, R-N.C. penned a letter to the CDC accusing the agency of putting "political obedience to Democrat-aligned special interest groups" ahead of "our nation's youth."  

The lawmakers demanded the agency release all correspondence "between or among employees, contractors and representatives of the CDC and representatives of any teacher's union" since President Biden entered office, citing emails that show regular correspondence between the government agency and the American Federation of Teachers. 

Fox News’ Yael Halon contributed to this report.