Twenty-one schools within the nation’s largest public school system will remain shuttered until their ventilation systems are repaired, according to New York City officials.

The nearly two dozen schools located within 10 separate New York City buildings were found to have faulty ventilation systems after an evaluation of 1,485 buildings was conducted ahead of the start of the new school year, a spokesperson from Mayor Bill de Blasio's office told Fox News.

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Since June 3, the city had been surveying, assessing, repairing, and improving buildings to guarantee proper ventilation amid the progression of the coronavirus pandemic, the mayor's office said.

The gate is locked at a public school in Brooklyn on September 01, 2020 in New York City.  (Photo by Spencer Platt/Getty Images)

The gate is locked at a public school in Brooklyn on September 01, 2020 in New York City.  (Photo by Spencer Platt/Getty Images)

A proper ventilation system is defined as a "free flow of fresh air in and out of a space, and can, in the vast majority of cases, be achieved by having an open window," according to the mayor's office.

As part of their efforts, the city had Ventilation Action Teams -- comprised of School Construction Authority (SCA) contracted third-party engineers -- comb through city buildings to determine whether the ventilation systems, both mechanical and natural, in rooms that staff or students might occupy are operating or in need of any repairs.

Until schools reopen for in-person instruction on Sept. 21, the city says it will prioritize the 10 buildings for "ventilation system and airflow repairs" and expects that "a number of these repairs to be completed within the next couple of days," the mayor's office said.

In the meantime, staff within the buildings will temporarily work from home. The city is also identifying alternative space for learning "if necessary" in the case that repairs are not made by the projected reopening date.

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Earlier this month, New York City had postponed students’ return to classrooms by more than a week to keep working on coronavirus safety precautions.

The reopening faced opposition by teachers unions, which were concerned over the city's push to reopen schools.

However, instead of starting a mix of in-person and remote learning on Sept. 10, the city’s more than 1 million public school students will start remote-only classes Sept. 16. In-school instruction will begin Sept. 21.

"The health and safety of our students and educators is paramount as we move toward reopening schools for teachers tomorrow, and students on September 21," a spokesperson from the mayor's office said. "With our infection rate consistently at or below 1%, New York City is uniquely positioned to reopen schools."

The mayor's office said its plan to reopen schools "with its extensive health and safety precautions and citywide testing and tracing program — consistently ranks as the strongest in the country and among the most rigorous in the world."

On Sunday, New York City Gov. Andrew Cuomo announced that the state had surpassed a 30-day mark where the infection rate was below 1%.

The Associated Press contributed to this report. 

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