Even beyond the absurdness of the comparison you are making (to an imagined best practice standard of isolation), in this you are simply being completely afactual or perhaps more accurately engaging in broad fantasy making about what happens during prolonged school closures.
Even for an influenza pandemic there has been sketchy evidence that school closures would be effective precisely because what you imagine, families all staying at home together huddled, parents not working or able to telecommute, no kids going over to each other houses to play, is widely understood to not be a likely reality.
School is not in session for most of the country now. Are parents all huddled at home with their kids across the country? Are parents all isolating themselves and their children? Is spread seemingly slowed down by schools not being in session?
Oh some parents will be unable to work because of this but many more will be interacting in the world and then having twice daily interactions with grandparents and others of greater vulnerability called in to help with the kids of all ages, and in ad hoc daycare arrangements, extended family and otherwise. Kids out of school will not be kept in isolation cells, and school closures will very possibly cause more exposures of adults with greater vulnerabilities to other adults with greater exposures, especially in the greatest risk demographics.
What you imagine is not even likely true for influenza in which kids are clearly more commonly infected than adults, much more contagious, and for longer periods of time. Kids are not kept home during closures and the impact of school closures even in the case of influenza while likely real has been small with little impact on adult rates.