On Friday, Yosemite Unified notified families of two positive cases of COVID-19 on the high school campus.
Staff members immediately took action, quarantining anyone exposed.
The district is not commenting on if the positive cases are among employees, but officials say those individuals were on campus Wednesday, August 11th - one day before the first day of school.
It's an unsettling reminder for some other local districts set to return to the classroom next week.
With students across California returning to the classroom, State Superintendent Tony Thurmond urged vaccines for everyone over the age of 12.
What we're seeing with the Delta variant is children are being affected, children are ending up in hospitals on respirators and worse. We want to prevent that," he said on Friday.
RELATED: Valley Children's Hospital seeing increase in COVID-19 patients
Another Valley district - Clovis Unified - returns to the classroom on Monday.
After much discussion - the district will require students to wear masks indoors in accordance with state guidelines.
A doctor's evaluation is required for any exemptions.
RELATED: Masks required at Clovis Unified schools unless a doctor signs off on student's exemption
Action News recently spoke with Valley Children's pediatrician, Dr. Hailey Nelson. She says there are very few recommended mask exemptions, but the list includes children under two and those with certain developmental disabilities. But for other students...
"This is a skill that will protect their health, that we can all work to provide that positive reinforcement and help our kids learn because that's how we're going to keep them healthy," Dr. Nelson said.
Clovis Unified did not have an exact number on how many exemptions are now on file, but they estimated about two dozen.
As for the 43 students in quarantine at Yosemite Unified, they're being monitored closely and continuing their studies from home.
The district also conducted a deep cleaning of the areas impacted.