After two postponements, New York City started in-person instruction for 300,000 elementary school students Tuesday, but on the same day the city's daily positivity rate for novel coronavirus tests hit 3.25 percent, and the mayor said he will close schools if the seven-day rolling average reaches 3 percent.

Mayor Bill de Blasio called the latest rate "cause for real concern" in a Tuesday news briefing. The 3.25 percent rate is a jump of more than a percentage point over the previous day. The current seven-day average is 1.38 percent.

Hundreds of thousands more children, including middle and high school students, are expected to be in classrooms by Thursday, but the positivity rate Tuesday was the highest in months and could lead to the closure of the nation's largest public school system.

Schools would be able to reopen when the weekly average positivity rate drops below 3 percent. Random student testing is set to begin Thursday, which could drive down the positivity rate.

New coronavirus clusters have been seen in parts of Queens and Brooklyn, including neighborhoods populated mostly by Hasidic Jews, who generally don't send their children to public schools and where mask compliance and social distancing have been lacking.

The city has threatened to implement tight lockdown measures to enforce mitigation efforts in those areas, including closures of nonessential businesses and religious schools if the communities don't take proper precautions. On Friday, the mayor ordered the city's police department and the New York sheriff's office to enforce mask mandates in the neighborhoods.

De Blasio said Tuesday that tougher enforcement, including steeper fines, would start "on a large scale" immediately.

"This is an inflection point. We have to take serious action and we will be escalating each day depending on what we see on the ground," he said.

The mayor had postponed reopening the schools twice because of objections from unions that the schools weren't adequately prepared to fight the virus and because of teacher shortages.

Michael Mulgrew, the head of the teachers union, on Tuesday expressed concern about reopening schools in Brooklyn neighborhoods with coronavirus clusters.

"We are very concerned with Brooklyn. If we don't see those numbers start coming down by the end of the week, we are going to get much more aggressive with City Hall," he said.

"Everything goes on the table. I know that makes people uncomfortable when we say those things, but it's a fact. We cannot, cannot allow politics to get in the way of our safety concerns at this point in time," Mulgrew said.

A survey of all 50 states showed that a total of 624,890 cases of the coronavirus in children were reported between the start of the pandemic and Sept 24, or 10.5 percent of all cases in states that reported infections by age, the American Academy of Pediatrics and the Children's Hospital Association announced Tuesday.

"At this time, it appears that severe illness due to COVID-19 is rare among children," the survey's authors concluded.

The survey's totals don't distinguish between the different age ranges. The authors also said it is not possible to determine the number of children infected but not tested.

The survey found a 14 percent increase in child cases in the two weeks ending Sept 24 — a total of 75,458 new cases.