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NEW YORK CITY — A day after New York City officially pushed back the start of in-person learning for most schools for a second time, a teachers union president explains the biggest issues behind the decision.

The city had initially planned for all schools to reopen on Monday, Sept. 21, but now only 3-K, Pre-K and District 75 students will return on that date. The rest of students will return on a rolling basis, based on grade, in the following weeks.

United Federation of Teachers President Michael Mulgrew told PIX11 Friday that inadequate staffing was the biggest concern in the days leading up to the planned reopening.

Mulgrew said he told Mayor Bill de Blasio on Wednesday, “you cannot open a school unless you have a teacher for every classroom. We cannot put all the children in an auditorium because that breaks all the safety protocols.”

The mayor said Thursday another 2,500 educators will be deployed to city schools, in addition to the 2,000 new teachers added this week. However, that 4,500 total still falls short of the 10,000 the principals union requested and the 7,000 to 8,000 the UFT has mentioned.

“We should be able to,” Mulgrew said when asked if the city can reach those kind of numbers by Oct. 1, when all schools are now set to be back open again.

Another issue has been at least 55 city school employees testing positive for COVID-19 in the weeks leading up to the new academic year.

Mulgrew said he believes that number could be higher because the city’s program for testing and contact tracing in the school system was not working. However, he said that is getting better after ongoing conversations and that schools are now getting test results back in 24 hours.

Mulgrew said there are still ventilation issues in some schools but that most of those have been addressed and fixed.

When it comes to personal protective equipment, Mulgrew seemed confident that schools were in a good place.

Considering the outstanding concerns, does he think there’s a possibility of schools going fully remote for the year?

“That’s really about the virus,” Mulgrew said. “If the virus spikes up, yes. That would be the determining factor to everything going remote … That’s the only thing that would cause us to go all remote.”

Mulgrew said he and the mayor agree that there is no substitute for in-person education.