CNN  — 

New York City middle schools reopened for in-person classes on Thursday as a surge of coronavirus cases that had shut schools in November continued to fade.

This is the second such reopening for middle schools in New York City, the nation’s largest school district. Schools were first closed in March 2020 when the pandemic began, but the city moved to reopen for in-person classes in September.

At the time, Mayor Bill de Blasio said that schools would close again if the 7-day positivity rate in new Covid-19 cases exceeded a 3% threshold, and in November, a surge in new cases led de Blasio to close in-class learning.

The city has since abandoned that 3% threshold. The 7-day test positivity rate for NYC residents is over 7% as of Thursday, though it has been declining in recent weeks.

The middle school reopenings are part of the city’s plan to prioritize in-person classes for younger children. Elementary schools reopened for in-person classes in December, while public high schools remain remote only.

Two key factors fueled the push to reopen school for younger age groups: Younger children have struggled the most with online learning, and younger students also appear to have the lowest rate of coronavirus spread.

In addition, teachers returning to work in person were prioritized for vaccine access at city hubs over the last several weeks. Teachers became eligible for the vaccine last month, and about 30,000 educators have been vaccinated since, de Blasio said Wednesday.

Michael Mulgrew, the president of the United Federation of Teachers, said the teachers union will be monitoring the reopening to ensure the city follows its safety precautions.

“These strict standards, and the requirement that buildings close temporarily when virus cases are detected, have made our schools the safest places to be in our communities during the pandemic,” Michael Mulgrew said. “They will continue to be the strongest protections for the health and safety of students and staff.”

CDC pushes to reopen schools safely

Mayor de Blasio greeted masked students and teachers with forearm bumps at Richard R. Green Middle School in The Bronx on Thursday morning. Signs saying “Welcome back” were taped to the outside of the school and students entered on a red carpet as part of the reopening.

The US Centers for Disease Control and Prevention recently released guidelines for reopening schools that focus on five key Covid-19 mitigation strategies: the universal and correct wearing of masks; physical distancing; washing hands; cleaning facilities and improving ventilation; and contact tracing, isolation and quarantine.

Vaccines and testing are not among the “key” strategies the agency lays out, calling them “additional layers” of Covid-19 prevention.

With elementary students and some middle school students returning to school, New York City public schools will have about 250,000 students in classrooms by the end of this month.

NYC Department of Education Chancellor Richard Carranza first announced the return to classes about two weeks ago. He said teams would be added to conduct weekly Covid-19 testing in middle schools, and weekly testing in elementary schools will continue.

CNN’s Elizabeth Cohen, Bianna Golodryga, Jacqueline Howard, Kristina Sgueglia and Elizabeth Stuart contributed to this report.