De Blasio, joined by officials from both the teachers' and principals' unions, delayed in-person learning in New York City public schools until September 21 with remote learning set to begin on September 16.
School was previously scheduled to begin with a blended approach on September 10.
"There is nothing more precious than taking care of the children of New York City," the mayor said. "Resolution has been found in a constructive spirit. We've come to an agreement to move forward."
City Council Speaker Corey Johnson, who has been critical of the mayor, said "Mayor de Blasio dragged his feet while parents and educators fretted about how to make the impossible work, waiting until a week before school is scheduled to provide clarity for our school community."
However, on Tuesday, Johnson said the mayor's decision to delay the reopening of schools was a step in the right direction.
WATCH: Mayor de Blasio's comments on delaying the start of the school year
Staff will report the day after Labor Day, and teachers will begin preparing their classrooms on September 10. Students will log on for remote learning for three days beginning on September 16, with blending in-person learning starting September 21.
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"Teachers who usually get two days of professional development at the beginning of the school year will now get nine," Schools Chancellor Richard Carranza said. "We've heard from everyone in our schools that have said we need some more time...This will strengthen and improve and make it so we have the safest start the school year."
Still, he urged parents and students to have "flexibility and patience."
WATCH: Parents react to delayed reopening of schools in NYC
There will be a monthly medical monitoring program for every school in the district, to be governed by health leadership, and the program will assess a certain number of kids in each school each month on a mandatory basis. There will also be a 30-day supply of PPE in all schools at all times.
"This is what I would hold up as an example of how to get things done," United Federation of Teachers President Michael Mulgrew said. "We now can say that New York City Public School system has the most aggressive policies and greatest safeguards of any school system in the United States of America."
WATCH: Teachers' union president endorses back-to-school agreement
Once in-person learning has begun, the DOE will establish a mandatory, robust system of repeated random COVID testing of adults and students. Under the new agreement, any student who refuses to be tested will be required to attend school remotely, and any staff member who fails to comply with mandated testing will be placed on unpaid leave.
Beginning immediately, prioritized access to testing will be available to students, teachers, and DOE school-based staff at 34 sites across the city. These sites will give priority to all DOE workers so they can quickly be tested, and provide results within 24 to 48 hours.
Then, beginning October 1 and recurring each month, it will be mandatory for schools to test a random 10-20% sample of their students participating in blended learning and on-site staff population. Families will be asked to sign a consent form at the start of the year for their child to participate at random and will be notified ahead of time if their child has been selected for the month.
"We think we're going to get a strong positive response," de Blasio said. "It's a self-swab test. It's a Q-tip, not the long apparatus that goes farther up your nose. I've done that one, I don't want to do it again. I like the Q-tip. I think parents will feel comfortable with that. It's free. We'll go about the process of getting those consent forms."
Mulgrew said the decision on whether to re-open a building to students will be based on the UFT 50-item safety checklist, including social distancing of student desks, the availability of masks and face shields, and a room-by-room review of ventilation effectiveness.
Buildings or rooms that do not meet safety standards will remain closed.
Students or staff found to have the virus, even in the absence of symptoms, will be quarantined for 14 days. City tracing teams will be dispatched to school immediately to determine potential contacts.
The presence of a COVID-19 case or cases confined to one class will result in the entire class moving to remote instruction, and more than one case in a school will mean that the entire school will move to remote instruction until the contact tracing is completed.
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Schools will need to switch to 100% remote instruction if the percentage of positive tests in New York City are equal to or more than 3% using a 7-day rolling average, but even if the overall case rates across New York City were to remain low, all school buildings could be closed if there were recurrent uncontrolled outbreaks in schools.
Additionally, any New York City zip code that reports a percentage of positive tests of 3% or higher using a 7-day rolling average will be saturated with additional testing and tracing including, but not limited to, increased testing of individuals in schools, opening new testing sites, door-to-door canvassing and targeted robocalls for at least a 14-day period or until such time as the 7-day rolling average for positive tests is below 3%, whichever period is longer.
Council of School Supervisors and Administrators President Mark Cannizzaro issued the following statement:
"New York City school leaders are among the most talented and dedicated in the world, and if given adequate time, resources, and discretion, they will do everything in their power to keep their communities safe and provide students with the high quality education they deserve. We commend Mayor de Blasio and Chancellor Carranza for making the right decision to delay in-person learning. The task before us is still monumental, so it is incumbent on the DOE to seize this time in support of school leaders so that these additional days will provide a much needed opportunity to implement necessary safety protocols, program classes, and align all school staff towards critical goals for this unimaginable school year."
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