Putting aside how long it took Walmart to require masks for a moment, the two situations really aren't compatible. A lot of people go in and out of stores like Walmart every day, but they don't tend to spend six hours there five days a week like kids do with school. It's easier for people to social distance within a store since they're just going in and out to get what they want/need. I haven't been inside a Walmart for at least ten years, but they're pretty big too. Everyone can wear a mask while walking a school hallway, but I don't know how effective they'll actually be if there are hundreds of students crowded together. Based on the schools I went to, which does include a private high school and college, there just isn't enough room for students walk around the hallways while being six feet apart. There's also the issue of where people still refuse to wear masks going into stores. I don't think I've heard of any stories of that being a problem in Walmart specifically, but wearing a mask is still considered controversial for at least some people when it really shouldn't be.
I haven't been on a school bus since I was twelve, but if they're still full of chaos now as they were back then, I don't know how well they can manage wearing masks, especially when buses are also pretty crammed. They didn't even have seat belts installed when I was a kid, although I don't know if that has changed yet. The spacing issue in classrooms is also a big concern, especially when classrooms are usually pretty small. To be fair, maybe the public school system in your area is different than it is in mine and I'm going off a lot just by what I remember of what my schools were like too. But based on what I've seen and heard, on campus learning is just not a safe and viable option. If they really wanted to have schools reopen in time for the fall, the situation should have been handled better from the start and schools should have gotten more funding to help cover safety measures as opposed to being forced to reopen now when the situation is even worse than it was back in March.
I'd like to address the "kids are gonna be kids and disobey" aspect of what you're saying. I think this is such a collective national event/trauma, there won't be a rebellion quite like what I think you're describing. I think a majority of kids want their schools back (I know mine were sick of home schooling from late March to the end of May earlier this year).
Plus, I think a majority of kids want to do "their part" to help fight the virus. They're going to conform, sit in a manner on the bus assigned by the drivers, they're going to proceeds through hallways as instructed, and abide by social distancing and mask wearing policies. If the concern is the occasional "my dad says the virus is being overblown, so I don't have to wear the mask", then that kid won't be allowed, they'll be sent home (or not allowed on campus in the first place). The rules are going to be strict, but I think people want this to succeed, until a vaccine is readily available.
I think this is going to be a large social experiment the country is about to undertake. We will see what states fair better this Fall. I think across the country, there are going to be schools that have serious outbreaks, there are going to be schools that perform masterfully and go by without incident, and I think there are going to be a majority of schools that fall somewhere inbetween.
We will see how it goes.