Good luck getting a bunch of 1st graders to wear masks all day and social distance.
Wearing masks will admittedly be very hard, but I don't think social distancing would be that hard to keep outside recess? Basically, sit students as though they are going to have an exam all the time.
Of course, keeping social distance in recess for 1st graders (or really any students) will be impossible.
If I was designing a plan for schools to open it'd be like this:
1) Stagger recess times even more than usual and divide the school's courtyard so that classes or grades are segregated. If necessary, cut recess times or outright cancel it and have it inside the classroom, at least for upper grades. Or maybe do it on a day by day basis (so 1st graders get it on Mondays, 2nd graders on Tuesdays, etc)
2) Sit students as though they were going to have a test the entire time. This probably is not good social distancing but it is as good as it is going to get.
3) If possible, reduce the amount of students in class. This is easier for middle and high schools since they can go half online, half in person if need be; but harder to pull off for elementary.
4) A variation on 3, if time allows, divide grades into more groups than usual. If there are other spaces in the school where classes can be had, use them. When I was in like 1st grade I had my classes on what can basically be described as a glorified closet at one point. Use laboratories, the cafeteria, gyms, sports courts and if weather allows, even have classes outright in the open in the school's courtyard.
5) If someone at the school gets COVID, close down the school for at least a week, test everybody (or at least as many people as you can) and what not. Basically try to avoid people spreading the disease.
All of this, plus cases going down with good leadership should allow at least a majority of states to open up their schools by late September. Of course, the COVID pandemic would need to have good leadership and go away for good.