I've been thinking about this kind of thing more and more lately. Following the news of countries that are doing better than us in keeping the virus in check, it seems they are forced to shut things down regularly. A case will pop up and restrictions go into place. This does not appear to be sustainable to me. They are playing whack-a-mole. I think it's only a matter of time before a "mole" pops up that they don't notice in time and infections are widespread again. Then, what do you do? Widespread lockdowns for months again?
Personally, I think the sustainable path forward is to learn and understand what level of infection is sustainable and can be kept in control by our health care system. Keep restrictions in place with the aim to control, but not eliminate, the virus. Then you don't have a need for hyper-vigilance at all times, possibly for years on end.
That said - as it applies to schools I agree with what you're saying. You can't shut down a school for one positive case. That's just not reasonable. You shut down a school if it's clear the virus has started to spread in the school. Maybe 5-10 related cases, or some other number. But if you have to shut down for a single case, nothing can ever be open. And - more importantly, you're not even working toward what your actual goal should be - preventing spread of Covid in the schools. If you catch one case, you send that person home until they're no longer contagious. It may be that they never spread it to anyone else. If they did spread it, then you go online only for a period of time.
Of course, I'm no expert but that is what makes sense to me.