Former New York City Mayor Mike Bloomberg called on President Biden to "stand up" to teachers unions that are stonewalling efforts to reopen schools. 

"It's time for Joe Biden to stand up and say ‘kids are the most important things.’ Teachers are going to have to suck it up, stand up and provide an education," Bloomberg, who ran for president in the Democratic primary, told MSNBC’s Stephanie Ruhle on Wednesday.

"The president has to stand up to the unions," he continued. 

To teachers who did not want to return to classrooms because they thought it was dangerous amid the pandemic, he argued incurring risk was par for the course. 

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"We have a lot of city, state and federal employees who run risks. That's part of the job. You run risks to help America," he said.

Bloomberg also called virtual classes "a joke, worse than a joke," particularly for the poor, who don’t have access to iPads, Wi-Fi or an adult able to sit down and make sure they are participating in class.

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As of Jan. 31, about 38% of schools only offered virtual learning, about 38% offered in-person learning every day and the rest offered a hybrid learning program, according to school reopening data tracked by Burbio.

Teachers unions are taking more heat from lawmakers and frustrated parents to get back to physical classes after the Centers for Disease Control and Prevention (CDC) released updated guidance finding low transmission rates of coronavirus in schools. 

"There has been little evidence that schools have contributed meaningfully to increased community transmission," it wrote.

CDC Director Dr. Rochelle Walensky said Wednesday: "Vaccination of teachers is not a prerequisite for safe reopening of schools."

Biden has for months campaigned to have schools reopen within his first 100 days, and his pick for education secretary, Miguel Cardona, led Connecticut's own reopening process as education commissioner. Cardona's Senate confirmation hearing is Wednesday. 

The White House insists testing capacity and safety measures are not yet robust enough to keep such a promise on reopening schools in just over three months. 

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White House chief of staff Ron Klain defended the teachers unions when pressed about the CDC study during an interview on CNN, saying there was a lack of funding for safety measures.

Anchor Erin Burnett asked him why "the unions in many cases are overruling what the studies show?"

"I don't think unions are overruling studies," Klain responded during the Jan. 26 interview.  "I think what you're seeing is schools that haven't made the investments to keep the students safe."