Four educators in Florida's second largest school district have died from Covid-19 within a day of each other, teachers union officials say.
The deaths in Broward County come as a fierce debate rages across the US over what virus protocols should exist in schools when the academic year begins.
Florida's Republican governor has threatened to withhold pay from school leaders who require masks for pupils.
The highly contagious Delta variant is spreading across a majority of the US.
Three teachers and a teaching assistant died between Tuesday and Wednesday in Broward County - which is just north of the Miami-Dade area. Their names were not released.
"Within a 24-hour span, we had an assistant teacher pass away, a teacher at her school pass away, an elementary teacher pass away and another teacher at a high school," said Broward Teachers Union President Anna Fusco.
"It's extremely frightening. I hear this every day. Whether in Broward or elsewhere."
Three of the educators were unvaccinated, said Mrs Fusco. It was unclear whether the fourth had received a Covid vaccine. She added that none of the teachers had been inside a school building recently.
The news comes as Broward County students prepare to return to classrooms next week. The district is the sixth biggest in the country, with over 272,000 pupils.
The Broward school board this week voted to require students to mask up in schools, defying Governor Ron DeSantis' order and his threat to withhold the salaries of leaders who do so.
The county is leading the nation in new Covid hospital admissions during the 3 to 9 August period, according to data from the US Department of Health and Human Services. Neighbouring Miami-Dade County is third in the ranking.
Florida and Texas together account for around 40% of the total new hospital admissions nationwide - and more younger people are needing hospital care due to the virus.
"Our numbers are horrible," Broward County Mayor Steve Geller said on Thursday. "The number of children cases are horrible and they are also rapidly increasing."
Showdowns between governors and local school districts over Covid rules have been playing out across the US ahead of the new academic year.
Another Florida superintendent cautioned earlier this week that two employees have died of the virus in the last two weeks, "and school hasn't even started".
Federal health officials warned on Thursday that 90% of US counties were seeing high transmission of the Delta variant.
Centers for Disease Control Director Rochelle Walensky reported the US saw about 113,000 new cases per day in the last week.