Chicago’s more than 640 public schools are staying closed until at least mid-April because of coronavirus worries, Mayor Lori Lightfoot announced Thursday.

The extension of school closures that were keeping the city’s 350,000 students home until March 31 further jeopardizes the remainder of an already-marred school year that saw a 12-day teachers strike last fall.

The school district, the nation’s third largest, won’t be back in session before April 21, Lightfoot announced in a televised speech Thursday evening that outlined the city’s continued response to the increasingly dangerous pandemic.

“We need to give parents and guardians plenty of advance notice about this reality and the ability to plan,” Lightfoot said.

WATCH LIVE: Chicago Mayor Lori Lightfoot delivers address on the city's coronavirus pandemic response Thursday, as the number of cases in Illinois climbs to 422. LATEST: https://abc7.ws/3dik1wO Posted by ABC 7 Chicago on Thursday, March 19, 2020

The mayor made the decision in consultation with city health commissioner Dr. Allison Arwardy and schools chief Janice Jackson “given what we anticipate as the continued upward trajectory of the virus spread.”

Gov. J.B. Pritzker said in his own news conference earlier Thursday that “parents should be contemplating the possibility” that the statewide closures will be extended past March 30.

Jackson wrote in a letter sent to CPS families Thursday that “while we want to reopen schools as soon as possible, this is a very serious challenge and we support the mayor’s decision to prioritize the health and well-being of our city.”

In a significant move, Jackson said the district is canceling district-mandated end-of-year testing, including the Northwest Evaluation Association (NWEA) MAP test, and teacher evaluations.

The NWEA MAP test, among other things, is used for entry to the district’s selective-enrollment high schools and elementary school academic centers. CPS said in a statement that “we are fully committed to ensuring students can access assessments tied to college entry, selective enrollment admission, and grade promotion, and we will keep families informed regarding any adjustments to the assessment calendar.”

CPS is also seeking permission from state education officials to maintain this year’s school ratings — suspending its School Quality Rating Policy (SQRP) — to ensure schools aren’t further negatively hurt by the closures.

Chicago Teachers Union President Jesse Sharkey said Thursday CPS has assured him that all teachers and staff will continue to receive regular pay through the remainder of the closures.

As far as homework goes, new state guidance this week said districts can grade students’ assignments as long as it doesn’t negatively impact a student’s overall grade.

CPS has already been handing out free breakfast and lunch to all city kids, giving out 532,500 meals since Monday in the first three days of the closure. Chicago businessman and billionaire Ken Griffin on Thursday donated $2.5 million — $1 million to CPS and $1.5 million to the Greater Chicagoland Food Depository — to help the effort, Lightfoot said.