The directive comes as state and local officials ramp up social distancing measures in an effort to slow the spread of the 2019 novel coronavirus.

Gov. Jay Inslee has ordered the closure of all public and private schools in King, Snohomish, and Pierce counties beginning Tuesday, March 17. Schools aren't permitted to resume in-person classes until after April 24.

"Although this virus presents much milder symptoms in children, health professionals have told us that closing schools could create a significant cut in the peak number of ultimate infections," Inslee said during a press conference Thursday afternoon. "And closing school districts will help slow the transmission of this dangerous virus."

Inslee had announced a ban on events attended by more than 250 people in the three impacted counties during a press conference the day prior. He'd also directed school officials to immediately start putting together contingency plans in the event of closures.

READ: Large events banned in Seattle area amid efforts to slow COVID-19 spread

The measure announced today affects 43 school districts and approximately 600,000 students — more than half the student population in Washington state. In addition to schools in King, Snohomish, and Pierce counties, Bainbridge Island, Stanwood-Camano, and Darrington school districts are also subject to the order.

"We are a visible observation for the rest of the country, and that's why we're taking significant steps to close schools on such a broad geographic area impacting so many of our students," said Chris Reykdal, the state schools superintendent.

Inslee said the order could be expanded to include other counties if necessary. The criteria for closures includes the number of confirmed COVID-19 cases in a county, the ability to trace the contact of people infected with the disease, and student and staff attendance.

King County has seen an 82% rise in student absences since the outbreak took hold, according to Reykdal. Snohomish and Pierce counties have experienced 56% and 13% increases in absences, respectively. Reykdal also pointed to a recent uptick in staff absences.

"It's been nearly impossible to have a continuity of services," he said. "And when you get to that place, you then have a very inequitable public education system."