Concerning Teachers: Teachers have a right to be concerned. A teacher in a middle school or high school may be in the same room as 100 different kids during the day. As of July 22, California had 413,500 confirmed Covid-19 cases, just a shade over 1% of California's population. The death rate in California for confirmed cases is 7,870, which is just a hair below 2%. California has about 300,000 teachers in K-12 public schools. There are more in private schools. Let's say that just 2% of teachers get the virus, and 2% die. That translates to somewhere between 6,000 and 12,000 sick teachers, and 120 - 240 deaths. Why should teachers have to take this risk when the general public does not?
Concerning Public Health: But the biggest danger is not to teachers. Kids with the corona-virus will more likely spread it to each other than to teachers. Those kids will go home, and some will spread it to their families. People say that we should open school "for the kids," because they seem to do better with the virus than adults. But that does not help Grandma or Grandpa. We have public safety laws to protect the public. We close schools partly to protect kids, but also to protect the larger public.