NEW YORK — Short of staff and supplies for opening New York City's public school system for both remote and in-person learning during the coronavirus pandemic, Mayor Bill de Blasio announced Thursday that in-school learning for most city students will be pushed back at least another week.

De Blasio announced a new timeline that will keep most elementary school students out of their physical classrooms until Sept. 29. Middle and high school students will learn remotely through Oct. 1.

"We are doing this to make sure all of the standards we set can be achieved," de Blasio said.

The plan is for the majority of students to be in school one to three days a week and learning remotely the rest of the time. About 42% of families have opted for remote-only instruction.

The delay came just days before students across the nation's largest school district were set to resume in-person instruction Monday. Now, only pre-kindergarten students and some other special education students will be going back into physical classrooms next week.

"It's not good enough because they're still just kicking the can down the road," said Daniel Leviatin, a fourth-grade teacher and school librarian at PS 59 in the Bronx. De Blasio has said reopening should not be a blanket decision but one driven by school or neighborhood data, Leviatin said, questioning what an additional eight days will change.

New York City schools shut down in March when the coronavirus pandemic hit and students went to all online instruction. 

De Blasio, who frequently points out that his own children attended city public schools, has insisted on opening schools in person this fall even as other big city school districts across the nation started the school year with online-only instruction.

But opening the massive system of more than 17,000 schools for both in-person and remote learning proved a daunting challenge.

Earlier this month, de Blasio delayed the start of school that was set for Sept. 10 to avoid a teacher strike and give schools more time to obtain personal protective equipment. At the time, de Blasio insisted there would be no need for further delays.

"We worked through the pieces and determined that this was a timeline that could address the outstanding concerns," he said. "It is a revision that still allows us to keep things moving forward on a tight timeline, but with additional preparation time. It's a good balance."

But on Thursday, the Democratic mayor announced another delay as he and union leaders, who had sounded alarms in recent days that schools weren't ready to reopen, said the city needed more time.

"We are doing this to make sure all of the standards we set can be achieved," de Blasio said.

United Federation of Teachers President Michael Mulgrew said it wouldn't have been safe to open all the school sites next week.

"If we are going to do this, we must make sure that we get this right," he said. "We want our school systems up, running and safe and we want to keep it up running and safe, because that's what the families, the children of this city deserve."

The unions had pressed for more staff as well as additional protective equipment and other supplies to protect against the coronavirus. De Blasio promised Thursday to hire 2,500 more teachers in addition to the 2,000 additional teachers he had previously announced.

"Our folks have been telling us, the teachers and the school leaders and all of the folks that are working in the schools have been letting us know that right now, currently, they are understaffed, they need some other items," said Mark Cannizzaro, the head of the Council of School Supervisors and Administrators. "Some schools you know need some more safety tools. It's varied across the city. But opening Monday to everyone would not have been safe for our students."