This comes after a sharp increase in the number of staff members and classrooms that had to be quarantined after exposure to COVID-19.
"This is heavily affecting our ability to keep students in school and provide services... Many of the staff and student quarantines could have been prevented if masks were required outdoors," the district said in a statement.
The new requirements begin Wednesday, August 18.
The district said they apply not only during classes but at all school functions (including extra- and co-curricular events and athletics).
The district said another factor that influenced its decision was the rise in local coronavirus cases reported by the Kaweah Delta Medical Center.
This story is developing and will be updated.