Well, kids are going to have a lot of substitutes for a lot longer if we aren't careful. More than a third of teachers are vulnerable to covid, so they may be out of the teaching pool to begin with. We will start to see outbreaks among the teachers who do start the year in-person. As that happens, other teachers will begin to balk in greater and greater numbers. The demand for substitutes, many of whom will themselves be unwilling to work due to comorbidities, will outpace supply. Schools will close due to these factors or due to uncontrolled outbreaks.
Schools do need to be open as soon as possible. But that requires planning many regions aren't willing to do, and that we aren't doing on a nation-wide level. It also probably required us to make some sacrifices - like opening up more cautiously, wearing masks more uniformly, and not opening theme parks - we weren't willing to make. You can't just yell, "OK, Schools open now."