Updated at 1:15 p.m. Jan. 4

It was not the back-to-school plan that many Massachusetts school districts had in mind.

Nearly a dozen school districts across the state that were scheduled to return from winter break on Monday remained closed or delayed reopening due to surging COVID-19 cases and concerns about testing staff before returning to in-person instruction. The Department of Public Health reported more than 31,000 newly confirmed cases of COVID-19 on Monday, which includes tests taken over the holiday weekend, lingering around the highest case counts ever in the Bay State.

With high case counts and state-provided rapid tests to distribute to staff, Boston, Brockton, Cambridge, Lawrence, Lexington, Brookline and Sharon all canceled classes for the day. In Boston alone, more than 150 teachers and staff reported they were sick. Even with rising cases and that staffing challenge, Mayor Michelle Wu said she remains committed to a Tuesday reopening.

“We are getting ready for the safe re-opening of school tomorrow,” she said during a press conference after the swearing-in ceremony of new Boston City Council members.

Boston Public Schools Superintendent Brenda Cassellius said the plan is to "assess each school case by case" with the goal of keeping schools open to maintain in-person learning. She added that an ongoing bus driver shortage remains an issue, but they have more than 100 drivers on standby, if needed.

Last week, the Massachusetts Department of Secondary and Elementary Education announced it had purchased at-home rapid antigen test kits, each containing two tests, to distribute to districts ahead of students and staff returning from the winter break. The department had previously acquired a supply of KN95 masks to hand out. Department spokesperson Jacqueline Reis said the state received 227,000 coronavirus tests from two different manufacturers and distributed iHealth brand at-home tests to more than 400 school districts, education collaboratives and approved special education schools.

But an apparent error resulted in some expired BinaxNOW brand tests being distributed to staff.

"Districts that have expired BinaxNOW tests were told not to use them," Reis said.

Boston Public Schools, the largest public school district in the state, received nearly 10,000 home tests. Spokesperson Jonathan Palumbo said the district handed out all the tests by noon, along with 30 KN95 masks for each staff member.

Palumbo said Boston Public Schools only distributed the tests the district received from the state.

"I have been asking around and it looks like some schools had a supply of tests from a previous shipment that we were not using," he said. "We have informed schools to not use those tests and are unsure why they were passed out today."

Other educators reported additional problems with the distribution, including receiving KN95 masks manufactured by Fujian Pageone Garments, which the Centers for Disease Control and Prevention reports tested at a maximum filter efficiency of 45.8%.

Reis said that earlier in the pandemic, the state coordinated testing of its KN95 mask supplies at an MIT laboratory. She directed GBH News to a document containing those test results found on the state’s website that shows 87.5% filtration efficiency for masks from the same manufacturer.

Staff also reported receiving single tests instead of a full box containing two kits, leaving them unable to confirm their test results.

Merrie Najimi, president of the Massachusetts Teachers Association, had requested that the state keep schools closed Monday to allow for COVID-19 testing of teachers and staff. She said the state's last-minute distribution of tests needed to be more organized.

“We are fully committed to in-person learning," she said, "but we fear the state’s dereliction of its responsibility in this case is recklessly creating this mass scramble.”

Speaking in Salem where schools were open today, Gov. Charlie Baker said it was critical for students to be in school as the school year continues.

“They do need to provide their kids with 180 days of in-person education this year," he said.

School districts made their own decisions on reopening and test distribution. In Salem, where students and staff have been undergoing weekly coronavirus testing for the past year, the state-provided rapid tests were distributed over the weekend. Superintendent Steve Zrike said the district is doing all that it can to keep students in school.

“There’s going to be some curveballs to navigate over the next couple of days, but we think it’s critical kids remain in school,” he said.

Brockton High School was closed Monday, and other schools in the district had delayed openings due to staffing shortages. All Community Schools programs, including the morning and afternoon sessions of Extended Day, are canceled this week.

Cambridge Public Schools are closed until Wednesday for student and staff coronavirus testing.

Chelsea schools reopened as planned with 74% student attendance and 93% of staff present Monday. Superintendent Almi Abeyta said the district distributed the state-provided tests on Sunday and mobilized quickly. She said they had systems in place and applied those methods to get tests to staff.

“We modeled it off what we had done before in 2020 with Chromebook distribution,” Abeyta said. “We’re like a well-oiled machine.”

This story was updated to include Department of Secondary and Elementary Education comment on the masks distributed to teachers and other school staff.