CNN  — 

As schools start classes across the United States, public health experts warn that the politics around masks could hinder the nation’s fight against Covid-19.

But the real-life evidence about masks leaves little doubt that they work.

In some places where schools have been open for a while now, such as Georgia and Hawaii, public health experts notice what happens when schools follow the science: classes go on without disruption as long staff and students wear masks.

But when staff and students do not wear masks, Covid-19 can spread, forcing people to stay home to quarantine and classes returning to virtual learning.

In just this week schools have closed, at least temporarily, in Georgia, Indiana and Mississippi due to cluster outbreaks among students and staff.

On Wednesday, in an elementary school in Cobb County in the suburbs of Atlanta, fifth graders were sent home for virtual learning due to high numbers of Covid-19 cases, according to a school district email sent to parents and obtained by CNN. Masks are optional for students and staff at the school, according to the district’s website, but there is social distancing in classrooms when possible.

“This morning, based on our district protocols and at the guidance of the Department of Public Health in coordination with district leaders, we’ve had to make the difficult decision to have our 5th-grade classes move to virtual learning due to the ongoing COVID-19 pandemic and high positive case numbers,” the email from East Side Elementary reads.

Students will be in virtual learning until Friday, August 20, and return to their classrooms the following Monday.