A photo went around on twitter of a supposed first day of school. Crowded hallway filled with teens. Don't know if it was genuine, but it doesn't matter because there isn't really any reason to doubt the point - if schools are open and kids are going to them, the hallways are crowded. There is no infrastructure for social distancing. Most schools have crowded hallways!
A lot of silly think pieces were published in the last few months, presumably mostly by people who haven't set foot in a school since they went to one, about possible social distancing strategies for schools. Separate the desks! Only 8 children to a classroom! Stagger opening and closing times! All fantasy measures, some of which might be possible in an alternative universe where schools had the resources to think about these things seriously, but the fact is... school hallways are crowded. Classrooms are crowded. While individual schools might try to figure things out, none of these solutions scale across the country without bumping into resource constraints. There aren't extra school buildings. There isn't extra money for the additional staff.
The fantasy is that outbreaks won't happen. Maybe a lot of schools will indeed get lucky. But... that's it. The plan is hope.