Mayor Bill de Blasio on Friday said that the city's public schools will reopen in September only if the coronavirus positivity rate in the city stays below 3%.

The threshold, which will be based on a seven-day rolling average of tests, is notably lower than the one set by Governor Andrew Cuomo and public health experts, who have recommended a cutoff of 5%. Citywide, the virus positivity rate has held steady between 1% to 2% for nearly two months.

As the largest public school system in the country, New York City is navigating uncharted waters as it attempts to establish guidelines for holding some in-person classes in the fall, all the while reassuring families and teachers that schools reopenings will be safe. The city Department of Education is expected to submit its reopening plan to the state on Friday. Students can expect to attend school between two to three days a week, depending on the size of the school and how many people opt-in for in-person learning. All families can elect to enroll for online classes only. Families must decide by August 7th, although students can pull out of in-person instruction at any time.

"We're one of the most densely populated places in the country," de Blasio said, during a press conference. "We fought so hard to come back from this disease. We're going to be very cautious to not let there be a resurgence. By setting this 3% goal we're sending a message: health and safety first."

But even though infection rates in New York have been among the lowest in the country, Governor Cuomo has said that an increase is inevitable given the number of out-of-state travelers.

At one point, de Blasio acknowledged the uncertain trajectory of the school year, in which schools might be forced to close several times during the year.

He spoke of the need to prepare "for a very non-linear experience."

"The school year proceeds over ten months and the fact is we don't know what that's going to look like for that whole period of time," he added. "Our hope and dream is a vaccine real soon real soon, everyone gets vaccinated, we're back to full strength."

Late Thursday, educations officials released a long-anticipated plan for handling COVID-19 outbreaks in public schools. The proposal, which must be approved by Cuomo and state health officials, calls for school closures in the event there are at least two cases in different classrooms.

Brooklyn Councilmember Mark Treyger, who chairs the education committee, criticized the plan on Twitter, calling it "an unfunded proposal that is incomplete." Treyger had released his own school reopening proposal last week, recommending middle and high school students stay home to remote learn.

In another announcement that is likely to draw criticism, the city said it will not mandate coronavirus testing for students and teachers.

Instead, officials will ask all teachers and staff to get tested before the first day of school and then at least once a month throughout the school year. The city plans to prioritize free testing for school staff with 24-hour turnaround time results at any of the 34 city-run testing locations.

Schools chancellor Richard Carranza, who joined the mayor at the press briefing, said he has been in talks with the teachers' union, which has expressed concern about the safety of their members.

"Now, do we agree on everything? Absolutely not. But we do have a process, where we're actually kicking the tires. And we're actually pushing the envelope," he said.

But following the press conference, Michael Mulgrew, the president of the city's teachers union, released a statement saying that the city's testing standards were insufficient.

"We need randomized testing of school communities throughout the year and a vigorous contact tracing system that gives schools test results and a course of action with a 24 hour turnaround," he said. "What's more, even if there are stronger safety standards in place, we still have grave concerns about the city's ability to enforce them in every school. Right now, this is not enough to protect students and staff."

As the pandemic surges across the United States, more and more school districts have chosen to move to online learning. Carranza said he has had conversations with his counterparts in 10 largest school districts in the country. Of those, only Chicago and New York City are considering holding some in-person classes.

Chicago education officials have said that the city's schools would end in-person classes if the rolling seven day average new cases per day reaches 400, or if there are 200 cases per day with “concerning epidemiological factors.”

De Blasio has resisted pushback on his plan, citing a survey that he said showed that a vast majority of families preferred having their children go to school.

"This is all about why do we even have a school system to begin with. To help our kids support our kids prepare our kids, especially after again, everything they've been through the last five months," he said.