If a single restaurant or gym remains closed for "safety reasons" -- even if every customer is a responsible adult who is free to make their own decisions about their personal safety -- then no school in that jurisdiction should open.
It is safe for schools to open because younger students are at a very, very, low risk for passing on the virus or for catching it. The risk of catching the flu is much higher and schools don’t close for that. I have no problem with teachers who are high risk because of age or underlying health concerns being permitted to teach online. But your average healthy adult teacher needs to get back to work.