The real answer is that schools probably should not open at all. They are basically ideal breeding grounds for the virus - kids are gross and spread everything to begin with, plus you have 30 kids sitting in close proximity, communal lunch-halls, crowded hallways, overcrowded bus transportation... and in higher grades you shuffle the contact groups an additional 7x per day as people change classes. It is literally the absolute ideal environment that you would design if you wanted to breed the disease - big groups, shuffled frequently, with no social distancing and kids who probably couldn't execute social distancing anyway.
The short answer is that schools probably should not be open at all. Even with the vaccine, you are talking about a disease that (even in mild cases) is known to cause subtle long-term damage to organs, it's really not a good idea to tempt fate in a growing kid, particularly with the potential for multiple recurring infections over time from different strains.
If you did want to re-open schools safely, you would have to require vaccinations, you would also have to restructure significant parts of the school "experience" - like having classes and bus transportation be significantly smaller and more spaced out, having class groups stay fixed and having teachers move between class groups rather than vice versa, etc. And all of that would be far more expensive - we would need more teachers, more classrooms, more busses, etc.
The reality is we're just throwing kids under the bus to die, because it would be expensive and inconvenient to do it right, and we can't not reopen schools because a lot of people depend on it as daycare and because a lot of people have decided we're "back to normal" regardless of what happens with future variants. Just like people have decided that we're done with masks and they're not putting them back on ever again, regardless of what the CDC says.
Open for business baby.