A total of 23 students and staff from two neighboring San Francisco Bay Area school districts have been infected by the coronavirus, the education institutions reported on Monday, shortly after the reopening of in-person classes.

 Last Wednesday, Brentwood Union School District’s 11 schools resumed classes and it has already logged 13 virus infections in elementary schools as of this week. Apart from this, another 10 cases have been confirmed in high schools.

None of the infected elementary school students have contracted the virus at school, Superintendent Dana Eaton clarified, saying that 55 students and a couple of staff were still placed on quarantine.

Eaton cited Contra Costa County Health Department’s warning to “expect positive cases in our schools.” The health department said that the schools’ positivity rate reflects the infections in their localities.

“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,” the superintendent said.

The third year of in-person education is being challenged by the increase of cases driven by the more infectious delta variant. It caused a division among parents regarding the precautions to be put in place, including the mask mandates.

Earlier in July, the Centers for Disease Control and Prevention renewed its guidelines and said teachers and students who were already vaccinated are not required to mask up indoors. It added that the distance of 3-foot is not anymore needed for those who completed their vaccine shots.

But in its general guidance last week, the CDC has updated its guidelines to recommend that everyone – regardless of vaccination status – should wear masks indoors.

Health rules in the state require students and the faculty to use masks without social distancing.

The guidelines will be followed in the schools in Contra Costa, school officials said, in a bid to stop the infection while maintaining the in-person instruction, KRON4 reported.

“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,” Brentwood Union school board president Scott Dudek said.