Those who state they want to re-open schools, but only when it is proven safe to do so offer an implicit lie, whether they know so or not.
The only way to determine whether something is safe or not is to try it. In fact, that's how we determine safe driving speeds. If we all drive 100 mph down the road repeatedly we know it's a safe speed regardless of what any speed limit sign says. We don't close schools for the flu until the flu in schools reaches a level of epidemic, which is that point at which infected students are infecting other students faster than students are getting better.
What they're really saying is they expect to have a veto power over what they alone determine to be "safe." They're not complaining about the potentially hazardous drive to and from school each day, which could conceivably hurt them and prove it unsafe. Did they apply the same standard for safety there, they'd never leave home. Worse, they're saying something like they only want to ride a bicycle when it's safe to do so without ever having made an effort to ride a bicycle.
It's pretty simple, and would happen to doctors and nurses did they adopt the same standards, as it happened to the air traffic controllers when they failed to show up. Send the kids to school and if and when the teachers don't show up replace them. We've an excess of young teachers looking for work and more than willing to start now.
Of course it's not clear that anything like a majority of teachers are saying this, rather only that proportion for whom union membership is more important than performing their profession. Open the schools and let whomever wants to sit home instead do so; that doesn't create an obligation that there will be a job opening for them later.
You are never called upon to prove a negative.
Amazing how Cartesian skepticism works only in one direction, you are free to invent any number of scenarios without evidence or reason to believe in them in order to try and create doubt.
But the concept of doubt depends on the concept of truth, a concept of proof requires a concept of validation. A theory of mistake also rests on a theory of knowledge.