BRENTWOOD, Calif. (AP) — Two neighboring San Francisco Bay Area school districts on Monday reported 23 coronavirus infections among students and staff, just days after in-person classes resumed. 

Students returned to classrooms at Brentwood Union School District's 11 schools last Wednesday. As of this week, the district has already recorded 13 coronavirus cases in elementary schools and another 10 in high schools, the Mercury News reported. 

Superintendent Dana Eaton said contact tracing has determined none of the elementary school students with COVID-19 contracted it at school. Still, 55 students and two staff members are now in quarantine, Eaton said.

Liberty Union High School District Superintendent Eric Volta said contact tracing was still underway for the high school students.

Eaton said the Contra Costa County Health Department has warned that "we should expect positive cases in our schools," and that the positivity rate at schools is likely to mirror that of their communities.

"Currently there have been 205 positive cases in Brentwood over the last 14 days, so it is to be expected that the schools will have cases," Eaton said.

The surge in cases because of the highly contagious delta variant already threatens to interrupt a third year of in-person instruction. Parents are divided over what precautions should be imposed, including whether to require masks. 

The Centers for Disease Control and Prevention updated its guidance the first week of July to say that vaccinated teachers and students don't need to wear masks inside, and 3-foot (0.9-meter) distancing of desks is not necessary for the fully vaccinated.

But the agency switched courses last week on general guidance, urging everyone to wear masks indoors, regardless of vaccination status.

In California, health rules require students and teachers to wear masks without social distancing. 

School officials in Contra Costa said they are following the guidelines to prevent transmission and have no plans to return any students to distance learning. 

"At this point here, we have no intention of going back to hybrid or anything like that, but obviously we'll be watching the numbers and that decision will be made," said Scott Dudek, Brentwood Union school board president.