Wow! This is the exact scenario that school staff is afraid will happen! All it takes is one irresponsible person, or even one person who didn't know they were or might be infected. Now what are these exposed teachers and kids supposed to do? Isolate from their families? Disrupt their entire households? For how long? How many people might they have infected without knowing? Getting tested the day after exposure won't tell the whole story. They'll need to wait a prescribed number of days before they'll test positive (if they contracted the virus). If only that person had kept their child home until their test results came back, a lot of people could have avoided the amount of anxiety, stress and possibly illness or worse.
Exactly this.
And here's the other thing, for all the people citing that schools need to open up so that people can go to work: that need isn't going to go away suddenly just because a coronavirus test needed to be taken. Delays in testing are going to cause parents to send their kids to school before the results come in. Just because you might not be a parent that does this doesn't mean that others won't.
People talk about how schools need to be open as if they're going to be open more than a few weeks anyway. And then what? We've spent all this time and energy focused on how to open, and not on how to do the remote that we know we're going to need to do at some point with fidelity.