NEW YORK (Reuters) – With only four days’ notice, New York City Mayor Bill de Blasio delayed the start of in-person learning at public schools for a second time for most students on Thursday as the city grapples to safely reopen during the coronavirus pandemic.

While virtual lessons via the internet are already underway, in-person learning had previously been delayed from Sept. 10 to Monday, Sept. 21, for those students who opted in.

Now, only pre-kindergarten children and students with special learning needs will start on Monday, the mayor said at a news conference. Elementary school students will begin Tuesday, Sept. 29. Middle school and high school students will start Oct. 1.

“This is a huge undertaking,” said de Blasio, who oversees the largest school district in the United States, serving more than 1.1 million children. “It is difficult. It’s challenging.”

Most other major school districts in the United States have scrapped plans to resume in-person learning for now. Efforts in New York City, which in the spring was the U.S. epicenter of the global pandemic, are being closely watched.

The mayor was joined by leaders of teachers’ unions, who had expressed concerns about efforts to mitigate the spread of the coronavirus. De Blasio has sought to reassure school staff that ventilation systems are being upgraded, nurses are being hired, protective equipment is being stockpiled and access to testing is being improved.

“If we’re going to do this, we must make sure that we get this right,” Michael Mulgrew, president of the United Federation of Teachers, said at the news conference.

De Blasio said a total of 4,500 additional educators have been hired.

The city had previously agreed with the unions that there would be monthly coronavirus testing of students and staff, with systems in place to send home classrooms or shut down entire schools if new COVID-19 cases are found.

(Reporting by Jonathan Allen and Maria Caspani in New York; Editing by Bill Berkrot)