Students enrolled in the Jefferson County Public Schools District between the ages of 3- and 11-years-old will be required to wear masks indoors at the beginning of the school year.
Students 12 and older won't need a mask, however, the district "highly recommends" unvaccinated students to wear a mask, district officials wrote in a newsletter to parents, faculty and students Friday afternoon.
Unvaccinated district employees are also required to wear a mask.
"Our primary goal in making this decision is to maintain an uninterrupted in-person learning experience for our students," officials wrote. "we simply cannot take the risk of extensive quarantines and fluctuating attendance which will keep so many healthy students out of school."
Charter schools within the district will have different mandates and will contact parents and students in the coming days, officials said.
Parents of students who have a medical or religious exemption to a mask requirement are urged to contact their school immediately to begin the 504 process.
District officials said they will revisit the requirement once the county's COVID-19 rate decreases to 35 cases per 100,000 people over a seven day period, or when the vaccine becomes available to children under 12-years-old.
"This decision will be revisited as we observe improvements in our case counts and vaccination status within the county," officials wrote.
Despite over 70% of the county's population being vaccinated, its case count has increased over 300% in the past 11 days to to 77 cases per 100,000 people, officials said.
On Wednesday, Jefferson County Public Health said in an email statement to The Denver Gazette that it supported masks as the Delta variant continues to spread in Colorado and across the world.
"Given that the more transmissible Delta variant is widely circulating in our state, we support mask-wearing as a precaution for anyone in indoor public spaces regardless of vaccination status," county health officials wrote.
Officials said despite Friday's announcement, there are "many moving parts" and changes could be made following "expected guidance from CDPHE" and the county health department.