The scheduled start of in-person learning at city schools has been pushed back, from Sept. 10 to Sept. 21, Mayor de Blasio said Tuesday.

Sept. 10-15 “will be devoted to preparation” by school staff, he said at a press conference. Sept. 16-18 will provide a “transitional period” for the school year in which online learning only will take place. Sept. 21 will mark the start of in-person classes.

“What would have happened on Sept. 10 now happens on Sept. 21,” Hizzoner said.

The change comes after the United Federation of Teachers threatened to strike after calling for a later start to the school year than the mayor initially proposed, saying adequate safety measures were not in place.

UFT President Michael Mulgrew joined the mayor at the press conference to voice his support for the plan.

“I can say to you now … New York City’s public school system has the … greatest safeguards of any school system in the United States of America,” he said.

The mayor promised monthly COVID testing for students and staff. The goal is to get “random” samples by testing 10-20% of students and staff at a time, officials said.

The testing will provide “really good insight into how many people may be infected at one time who do not have any symptoms,” said Dr. Jay Varma, the mayor’s senior advisor for public health.