New York City is again changing course on the reopening of schools.

New York City Mayor Bill de Blasio (D) on Thursday announced that the start of in-person classes for most public school students in the nation's largest school district has been delayed again, The New York Times reports.

The city previously set Sept. 21 as the day when in-person classes would begin, but de Blasio said the district will now be taking an "updated," phased approach to sending students back. The first phase will be on Sept. 21, when pre-K and advanced special needs students will return to school, per the Times. But elementary schools are now set to reopen on Sept. 29, and middle and high school students are going back on Oct. 1. The city had already delayed the start of in-person classes to Sept. 21 from Sept. 10 as part of a deal to avoid a possible teacher's strike.

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

The Times' Eliza Shapiro expressed shock at the last-minute, second delay that came only three days ahead of the previously-set reopening date, tweeting, "This is just astonishing." Brendan Morrow