The number of teachers calling in sick was more than twice the number of previous days this week.
Dozens of teachers used the opportunity to stage a drive-through protest in front of school district headquarters in downtown Oakland. They say they don't feel safe because of COVID.
"With principals out, case managers out. Teachers out. It creates an unsafe situation at our schools to have that little supervision at a time that is this severe," said Catherine Priestly, a science teacher from Fremont High School.
"This is something that was unapproved. It was unauthorized and technically illegal. It is not supposed to happen," said district spokesman John Sasaki.
Of the 12 schools closed for instruction, nine were high schools. Parents of younger children had to stay home or find daycare.
Among the school cancellations were Oakland Tech, although students said a handful of classmates were inside along with a few teachers.
"It's been weird. There is no one in the halls at all. And all the doors are locked. It's really quiet," said sophomore Evaline Flaymer.
The teachers are demanding the district give out KN-95 masks to students and staff, more sick leave, and remote learning.
The district says it is not considering remote learning. But it is in the process of giving out the masks. It is granting more sick days to teachers and staff.
It feels like a wildfire. We are trying to get out ahead of it. We are all on the front lines being with students every day. Seeing the mask situation. Seeing the spread in the community," said elementary school teacher Tamara Henry.
There is no indication the sickout will extend beyond Friday.