UKIAH, Calif. (AP) — Schools in one California county were forced to close this week when state officials said their coronavirus cases had been higher than previously reported, and several hundred students were sent home from class, officials said Friday.

Michelle Hutchins, superintendent of schools in Mendocino County, said she learned this week the county had been placed on a state monitoring list for coronavirus infections with a date retroactive to July 25, and that as a result schools should have not been allowed to open for in-person instruction after that point in time.

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Hutchins said on Wednesday she sent home 300 to 400 students who had already started the year off in largely rural schools. It’s possible elementary school students will be allowed to return under special waivers offered by the state but middle and high schoolers will remain on distance learning until virus conditions meet the state-mandated thresholds for the Northern California county to fall off the state’s monitoring list, she said.

“Now, they’re scrambling to not only switch to distance learning but to complete this waiver process,” Hutchins said, adding that most of the 12,500 K-12 public and charter school students live in more urban areas that had already planned to start the year with distance learning.

The situation comes after California faced a problem in late July with electronic lab reports that were getting backlogged at the state. The state said it has since fixed the data issue but it led to a delay in reporting about 14,000 virus cases.

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California students are facing a wide range of situations as they head into the academic year. Many of the state’s largest counties remain on the monitoring list due to higher rates of infection, and are only offering distance learning. Some are allowing elementary schools to apply for waivers from county and state health officials to reopen — something that so far has been obtained largely by private schools and a few small public school districts.

Some schools may soon offer in-person instruction in counties that have seen a decline in cases, such as San Diego County, which is no longer on the state’s list but must remain off for a total of 14 consecutive days to do so.

California Gov. Gavin Newsom issued rules last month that shut nearly all the state’s K-12 schools for classroom instruction at the start of the academic year. Schools in counties on the monitoring list can’t resume in-person learning until their county is off the list for the 14 days, except for those that obtain an elementary school waiver.

Once schools reopen for in-person learning, they will no longer be forced to close if the county where they are located returns to the list, the state’s Department of Public Health said in a statement. Schools must close if they report more cases or an infection rate among teachers, staff or students exceeding state-mandated thresholds, or if a local health officer determines they should.

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State health officials said they are allowing a three-day grace period for elementary schools in Mendocino County to remain open while they apply for waivers to avoid interrupting instruction. Hutchins said she already sent students home and was informed later of the grace period.